Hi
I was trying to find anything for you.
Well, if you have WarpUP there is DevKit with includes.
Inside powerpc_macros.i and powepc.i you can find that:
r0 = trash
r1 = stack
r2 = base (I _think_ its base pointer for data)
r3 = library base (I guess)
r13 = local (what the hell???)
there is also convention for 68k<->PPC registers mapping in case 68k calls PPC and PPC calls 68k
Hope it helps a bit for the beginning.
Good luck
PiR