It's easy in theory, but there are no documented details on how to do it. It's easy to say "just add a ROM", but how the heck does it get run?
You can either overlay your bootrom so it gets run before kickstart or put rom at 0x00F00000 (I think). From here you can add resident modules in ram that will get picked up later on.
It's possible to overlay the autoconfig space with your own hardware until it's configured and then pass the reads/writes on to the main board. It's probably harder to do this though.
You could get the board to write resident modules to chip ram.
If you're really crazy you can temporarily overlay part of kickstart rom so that when it scans for resident modules then it finds yours too. However you may have compatibility issues with different versions of kickstart.
