Hi All,
I can't seem to find a definitive answer to this. Is encumbrance supposed to be taken as a penalty to casting? None of the formulae mention it, and there seems to just be a single reference to it on page 178 of the core book. I'd have thought it would be called out more often.
Thanks for any input 
Yes, I think - page 178 is fairly explicit.
I haven't done a lot of searching elsewhere, but I think it's not dwelled on because it's assumed that most magi will deal with this by just not carrying heavy things (and in particular, either not wearing armour or magically lightening it as per the spell "Hauberk of Sublime Lightness on page 38 of HoH: Societates). Hugh of Flambeau in Magi of Hermes is listed as having a -1 encumbrance penalty, although the book isn't explicit on it applying to his casting.
Metacreator does not factor Enc into casting totals. It seems it should, per the reference made by an earlier poster, so just a heads up
Explicit indeed: "most athletic activities are penalized, as is spellcasting".
Magi aren't so assumed to carry light stuff: think that many magi carry enchanted staffs around, but staffs have a Load of 2 (which we houserule to 1, as Spears get 1, after all, and may be seen as staffs with a pointy end on the top, but anyway), which means Burden 1, and since most magi also have zero or less Str (with these characteristics points spent massively in more useful characteristics), that means that any of these staff-welding-magus will have a -1 casting modifier right in the face.
I'm ok with that anyway: after all most magi do invest in Sta, which adds to the casting roll, so you can see the whole picture as if magi carrying heavy stuff will suffer some disconfort from the heavy, bothering equipment, that at the time of casting will be countered by their usually high stamina.
Tell Alter Ego games?
Ah, but that's the reason why you do not cast wiggling your staff around as if it were a wand, but by planting it firmly on the ground, and possibly leaning on it -- a bold reminder to the SG that you are not holding up the staff (at most, the staff is holding you up) so it does not contribute to your load! 
Good solution. I can totally understand that, then, you get the penalty only if you start hitting people around with the staff (this is, you are using it as a weapon), but you don't if you use the staff to help you walk though rough terrains.
Thinking more about that I guess it could be houseruled something about load when you are not actually wielding something. There should be a reason why warriors used to walk around carrying their shields and bows hanging on their backs.
Thanks for the confirmation, all.
I just grabbed the Metacreator bundle and am over all super happy with it, so it's good to have a heads up about that, too. Now if I can only find interested players.
The other bit you'll want to know is that Metacreator uses a distinctly non-canon version of Affinity. It's not drastically off, but it is off.
Yes, indeed. With MC, if a character acquires an affinity after character creation, adding the affinity is effectively applied retroactively to all experience in that Art or Ability. So, if you had 50 xp in an ability, under RAW when you add an affinity his ability score is still 4, but in MetaCreator, if you apply the affinity his score becomes a 5 because it lowers the XP level by 2/3. That's a pretty big difference. I do recall getting him to change his implementation of Flawless Magic, because he used the same method of lowering the XP level required by half, rather than doubling the XP. I'm not sure why I didn't request him to fix Affinities at the same time. If it's important to you, you might wish to consider it.
Yes, I should, shouldn't I?
You should