Actually you can go for 64 colours (half of them are shades of their parent colour though), I did for AP2, if you really want ocs, also you should split the interface to be on the bottom or top of the screen and you can change the complete palette for that, only the pointer colour must be shared, the rest can change within the very same screen easily. Also instead of using gradients in the gfx (sky etc) you can change some selected colours on each scan line. You can also change the palette partially if you really want to, but that will take away most of the cpu cycles when the copper takes the bus.
...but why the hell would you like true colour gfx converted to ocs anyway. It will look just sad, since this game is designed for true colour, not something you created from scratch keeping possibilities in mind.
People who are not just on a nostalgia trip with the miggy do have gfx cards.
If you want to make real amiga users happy, make an sdl port and make it compile on mos and os4.
People who use emulated amigas emulate them on their pc and mac, and they can go and play these games natively, in full colour with surround sound and extremely hgih resolutions. So don't bother with just that.