A G3/G4 A1200 PPC card makes absolutely no sense. It makes "slightly" better sense for a big-box Amiga, even then I highly doubt there are even 1000 A4k owners who would be interested. (Actually, it sounds like a slow, expensive, hacked-up architectural mess.)
Best option for OS4/MOS/AROS users = modern or semi-modern PPC/X86
Best option(s) for "classic" 68k Amiga users = emulation, MiniMig, "Real" HW or possibly NatAmi
The only type of card that *might* sell OK IMHO would be something like an '030 A1200 card. I'd bet there are far more people out there who would spring a buck-fifty to run WHDLoad comfortably on their old A1200s, as opposed to those willing to spend $1000 to run OS4 on their _old as hellz_ A3000, especially at the expense of games/demos/WHDLoad/etc.