QUOTE (Udoshi @ Dec 7 2010, 03:08 PM)

The answer is no!
Here is why:
Q: Ally spirit powers can be chosen from any powers available to spirits the initiate's tradition may conjure. If the initiate knows Invoking metamagic, are the powers available to great form spirits available as ally spirit powers?
A: No. The powers must come from those available to the initiate's tradition; Invoking is not inherent to a tradition.
However, its my -opinion- that since ally spirits count as spirits of all spell categories(for purposes of, say, assist spellcasting), then they should count as spirits of the tradition-appropriate type for purposes of great form powers.
Its also my opinion as a -gm- that access to a great form power of your choice with unlimited services would be
broken as hell. I'd probably allow it, but at a bare minimum, make it cost a use of your ally spirit's edge. This would put a hardcap on the amount of times it can be used, and make it more of a special occasion thing when a spirit allows its magician to spend its edge.
I like your thinking. Balanced and fair.
You can also do it this way, by the book: let your Ally Spirit go free, Bind him as a free spirit, and then use Invoking. I doubt it would get any Great Form Powers, but the extra reach and whatnot are awsome (and actually, I'd prefer the "All LoS = LoS(A)" over Great Form Powers anyways).