An idea came up some time ago. Could a magus actually cast a spell onto his own Parma Magica? Let's say he's a Flambeau, and he wanted to cover his Parma Magica (which if I understand it correctly, is a globe around magus) with a shield of fire. So he actually uses the Parma as the target, but he's not trying to breach it. Could this work?
Eirik
Never thought of that before. I suppose you'd get the same effect simply by casting coat of flame on yourself without penetration, but then perhaps thats too iffy and most SGs would balk at the idea of the spell not simply fizzling against the Parma.
Thus, perhaps a viable solution would be to decree that the spell must be Range:Touch and Target:Part WITH a Vim requisite in order to target the Parma itself.
How would others feel about that?
I believe that it is not a proper globe. It instead covers the magus and most of his or her possesions as if he or she had been dropped into a tub of warm peanut butter and then removed (my analogy not from the book).
Yeah, I've always thought of it as a second skin of sorts as well. The best non-tailored suit, next to the one you were born in, that you will ever get! 
You can do a sphere, or coat of fire using the CrIg guidelines. Add a magnetude and a Rego requisite to not be burn yourself, at range personnal, and you got something similar of what you want.
If you really want to do as you suggest:
I remember no rules about targetting the Parma itself. Because you need to bypass your own Parma if you cast non-personnal spells, I would suggest that any spell to affect the Parma need to penetrate it. (See Winds of Mundane Silence, PeVi)
To say otherwise would mean that any one else could set another magus' parma aflame without the need to penetrate !
Then, if you can penetrate it, you can do what you want with it.
I see no need to add a Vim requise for the spell (does a Pilum of Fire need a Corpus requisite to be cast on a soldier?)
Notice that you need to sense it somehow to affect it with spells. Also, the Parma is a magical effect, it does not fuel fire, so your spell need a longer than momentary duration.
This an original idea... !
Maybe you could set on fire the mystic cord between you and your familiar, burning anything in between ?...
Casting a spell on top or outside of your Parma would IMO have a Touch range, meaning you'd have to penetrate. However there could be a few variations that might allow it.
A spell designed to do specific things can sometimes bend the rules a bit. Such a spell like this would have Touch range and maybe an extra magnitude for the special effect, but once cast would be on top of the Parma. It also might require a Finesse roll, which might be easy or not depending on the special effect put into it.
However, something like this could certainly be a Minor breakthrough. In True Lineages there are several variations on Parma protection, something like this would be of some interest to the more martial wizards. Being able to have a damaging aura that doesn't effect the caster could have some benefit. There are already spells like this, but don't necessarily offer the caster any protection. In such a situation the Parma wouldn't protect but the caster's Form would.
Hmm you employ a faulty comparison and then actually undermine your own argument and reinforce mine. Allow me to elaborate...
The PoF example fails as a comparative since in the case of targeting the Parma (as you go on to suggest yourself) you are targeting a magical effect (or more rightly the effect of a Hermetic Ability), not a tangible entity or form as in the case of the body of the soldier.
Thus, rather than simply adding some ambiguous magnitude to account for "special effect" as you and the poster above suggest, why not call a spade a spade and require a Vim requisite which would itself impose that needed extra magnitude and would account for the spells ability to attach its effect to a preexisting magical magical effect?
More than anything else, this strikes me as a breakthrough in the application of Muto Vim.
I think I understand your point, but I still think Ferrezt's idea is still worthy in his own right. If you would be as so kind to let me explain how I see it:
Where is it written that a CrIg spell need a tangible support to exist ? Given sufficient duration, would not a CrIg last in mid air ?
I think that is the heart of the intuition of Ferrezt, as I understand it. One could cover a magical effect (magical effect, spell, ot whatever, as long as it can have recognisable boundary) with fire.
I said there was no need for a Vim requisite, based on PoF. A spell like Blade of Virulent Flame (CrIg15, ArM5 p.140) would have been more appropriate. (But it's still a good exemple ! Arc of Fiery Ribbons, for exemple, should not be Target: Group, but should have +2 size, since it's really one fire created that is the target, not many persons...)
It cover a metal blade with flame and has no Terram requisite.
I used the rational of this last spell to cover with fire a magical effect such as the Parma. The Base GuideLine for +5 damage is 5. ("...in unatural hape, such as a ring, ..."), the same guideline as for BoVF.
I think that a Rego requisite (w/ +1 mag.) to make sure the flame doesn't turn inward is reasonnable. (see Coat of Flame, same page) ( Maybe the req. is not needed if the effect doesn't penetrate the caster Parma?)
I don't see the need for a breakthrough for such an effect, since it could be allowed by the RAW, and doesn't seem to damage game balance.
It's just...original =)
But for now, I see no difference between such a spell and one another how would simply create a sphere of fire around the caster, for exemple.
Maybe Ferrezt could explain ?