MorphOS and OS4 didn't really support the PPC Amiga accelerators. I mean they ran on it... sort of. Like a demo.

While OS4 (4.0) will have access to AGA, you might find it incompatible with the 4.1 most of the software is now built for. 4.1 never ran on classic, AFAIK.

The best use for a BPPC/CPPC setup is still to use AmigaOS3.9, WarpOS (with PowerUP emulation). Unfortunately you don't get later developments like MUI4. The retro boom unfortunately didn't result in a boom in 68k ports - that side of things is still dead as a plank of wood. Maybe once the litigation calms down, who knows...