I was wondering: could a magus with Muto/Rego Auram trap an "incipient" lightning bolt during a ntaural storm transforming it into, say, a small gemstone (a Conc duration effect produced by an item that maintains concentration), and release it when he needs a "natural" lightning to bypass parma? Could he temporarily suppress the force acting on a massive stone thrown by a catapult/trebuchet, while turning the boulder into a small pebble (a Mu/Re Te effect) - again canceling the effect when the pebble is thrown at an enemy, who is then hit by a full force catapult boulder?
Sure, but keep in mind that you would have to aim that lightning bolt, which requires a Finesse check.
Postulate:
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Natural lightning bolt, magically transformed into a gemstone by a Conc duration, item maintains concentration effect. Item, and gemstone, being carried by a magus.
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When transformed back, the lightning bolt flies straight out (let's say "in the direction of the flat facet" of the gem) and hits anything in the way. Requires a Finesse roll to aim.
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Now suppose that the item that's keeping the lightning transformed into a gemstone also has several other effects:
First Possibility:
Second possibility:
Thoughts?
Another possibility would be an item that turns lightning bolts into javelins, with a duration (using the minor illusion mystery "spell timing") "until the javelin is thrown and its motion stopped" (equivalent to sun, though it expires after at most a month). Useful for equipping grogs to fight against dragons, and the magus who uses "Wielding the invisible sling" can't miss with it.
I could see the lightning being done easily along the same guidelines that is done for the muto ignem spell: Trapping the flame. Just scale it right for muto auram.
That was, indeed, the inspiration. However, a lightning bolt is an entity that does +35 damage. You'd have to build a really hot fire in order to have a source of natural +35 damage flame. Lightning bolts are considerably easier to harvest- Cr or ReAu to make a thunderstorm, InAu and InAq to make your vision unhindered by clouds and rain, and Sight range Mu(Re)Au(form) to turn the lightning bolt into something and move it to your hand. Easy "harvesting".
I can see a magus with a necklace, a chain enchanted to keep the stones strung on it transformed into stones. Walking around with 30+ natural lightning bolts strung on his neck.
...and I can see him get hit with a big Unraveling the Fabric of Auram....
I like this idea. 
A character of mine once used a duration ends spell:
He turned fire into wood, then placed the wood on a pile of wood in the bad guys' castle and got himself an alibi. When the sun set, the spell ended...
I would say there is one problem though, unless the lightning can be discharged right on the target, basically touch range(which could also be a nasty "shock" for whoever tries it), natural lightning will simply NOT hit what you want unless its aimed, and if its aimed, i would say its no longer natural.
A potential way to get around it might be that the item is made to discharge when hitting something after thrown... Still requires a successful physical attack then, but alot nicer and safer to "deliver" on a target.
Are we assuming that since the released lightning bolt is no longer magical, it bypasses parma, or does the fact that it is directed (in the same sense of rego-ing a rock at someones head) mean parma applies as normal.
If parma applies, not so good for fighting dragons.
what you could have hear is that the Gem has a few effects
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Concentration duration convert the lightning bolt into a species of the Gem say a gleam in its construct (this is highly unnatural and approrpiately high level but I'm sure some other appropriate emans of trapping the item could be found...what the heck)
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Maintianing the demanding spell, Concentration duration (item keeps concentration). This means that with a moments thought you disactivate the second spell which disactivates the first causing a purely natural bolt of lightning to leap out.
it would need targeting but the lightning WAS natural and is now natural again and bypasses parma.
The game design intent has been explicitly stated by the line editor: you need to either require a Finesse roll, or allow magic resistance.
As I see things, if you have an effect that makes something harmful when canceled then:
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if you rely on a linked trigger, it fires (or fires at the right time) only if MT is penetrated. (e.g. a boulder might only grow back after it has already fallen on the ground).
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If you cast a spell to dispel the effect, you need to penetrate MT with your dispel. Or a roll might be needed to dispel at the right time (fast-cast?)
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If you are concentrating to maintain the effect, a roll might be needed to do so at the right time.
The game design regarding "aim or spell resistance" is for spells - not every last effect. One does not need to aim a landslide, nor a flood.
Without researching it, I'll suggest that lightning is viewed and understood as it was by most any of us when we were children, or as depicted by the Greeks - directional, as thunderbolts, javelins of lightning from the heavens. So, if that object was in a magi's pocket when the spell ceased, it might have about a 50/50 chance to go towards him or away from him - depending on the exact location. And then it would "natural" lightning - scary.
The "now the natural lightning is released" effect would not necessarily need to be aimed if you could get the object that holds the lightning onto the intended victim. It either goes toward the victim, or not - up to chance, but as unavoidable as a stampede if it's coming your way.
Even assuming some item that was marked "this side down" that held the natural lightning, I'm not sure you could guarantee any sort of aiming with any reliability. Lightning is traditionally represented as jagged lines - quite unpredictable in its exact path. A rough direction, "away from me" might be achievable, but hitting a target at any distance would be quite a stroke of luck - so the close, the better.
I'd have no problem with this as SG - if a mage can trap lightning in an item, that spell is plenty large enough to lead to the injury of a magus, no complaint from me, especially if the caster can get the item onto the person of the victim.
err - technically, by the rules you do, but since the distance off from target is measured in single paces per the amount the roll fails, then for those spells where the Size is "huge" it just doesn't matter if you are off by 10 or so!
Consider CrAu "mist" where the basic size is a cloud the size of a standard Boundary = 100p across.... If I miss my Finesse roll by 1, the centre is 1p away from the desired spot; if I miss by 10 it's 10p away from the spot... I still cover the central spot.
(Having said that, many magi like to cast CrAu "Mist" so it "just misses" themselves - and the Finesse Aiming roll rather more matters then...
Horseshoes and handgrenades and tactical nukes - sure, whatever. 
Hand grenades have been used IMS. Trapping the fire spell turned a large bonfire into a small stone. Throw a few of them. Get a little bit crazy with disenchant (if it was an actual stone) or WoMS and there you go with napalm-like explosions
We banned them after a while, but during a few months they were the default solution to combat. IN a few cases they were even cool and climatic
Careful about this disenchantment system of casting spells. We found it broke down the consistency of the system and our suspension of disbelief extremely fast. IN small quantities is good. Too much of it and the gaming world becomes a parody of itself.
Xavi
For added fun, research a non-Eternal duration Waiting Ward, possibly with Sun duration. Cast it on sling-bullets with the release condition being "When I hit my target." Throw your most powerful combat spell into them. Then fly above the battle field, invisible, and throw them about with ReTe magic. Instand, cheap 1-shot delayed fireballs or whatever you desire. True, they only last one battle, and disposing of excess munitions is, frankly, rather dangerous, but it does work.
For more fun put moon duration waiting wards on some mice and release them into your opponents castle suitably loaded with spells to start fires and destroy stone and wood. Just make sure you're got a nice vantage point for near the end.
Ouch!
The best I've done is shrink a monstrous lizard down with season duration (4th ed), and then arrange to have it sold to a mage who had a penchant for collecting magical animals. I knew he was busy that season, and so could not fully investigate the critter. Good times.
Get Tethered Magic or Mutantum Magic and you've got all that for free, without having to make sure that both the Watching Ward (for trigger) and the bound spell do penetrate.
For the mice... yeah, ouch. Also, how would you handle it if there is an Aegis in the way?
True, but the spell approach can be learned by anyone, one way or another, and allows you to deliver Touch spells at range. As far as the Ward goes, it shouldn't need to penetrate - the trigger conditions, as I understand them from reading the spell, are the same as those for triggering an enchanted device. As such, "when striking a target" is perfectly valid, and doesn't need to penetrate. An Intellego spell so that it went off "When Simon ex Criamon is hit" would need to penetrate, however.
Same as an enchanted device, which I believe is broadly like a Might bearer's powers - penetration loses the score of the Aegis halved. The spell description is a little vague on that topic though.
Nah....that ALWAYS goes wrong... the mice inevitably get somewhere you don't want them to be.... like inside a lords castle, your grain silo or somewhere just as nasty.
Playing a Mutantum magus, I can assure you that the Spell-jar type spells are always good fun when you can dismiss the spelljar and will and BANG. However how long does the spell jar last for and what happens when you casually discard them.