Regarding the MIPS(el) part:
I tried to make the MIPS implementation in Rockbox as general as possible, so all you should need to do is make a crt0.S, add you CPU to config.h as MIPS and that's it I think. (kernel.c and other files should be set up for MIPS).
If you would have any questions regarding MIPS in Rockbox, you can always ask me