On the topic, years back when I used a Mac constantly I considered some kind of hack that would patch the main GDevice to be 1/2/4-bit, then in a vertical retrace task remap it to the video card's 8-bit buffer every frame behind the OS's back. Since you usually run those old games at 640x480 or smaller, and this issue was present on fast PowerPC machines, they would have been plenty fast to do this. It'd be somewhat a pixel depth emulator, giving the OS a virtual screen.