I've been working on this for fun, although I think others could do it better.
This will eventually load images (or maybe boot code) from SD to HUB RAM
Right now, it does everything that "mount_explicit" does in FSRW.
What's left isn't very complicated.
This is rigged for P123-A9 with the uSD card on my 4.3" LCD adapter board, but setttings can be changed.
It outputs a lot of info about the SD and FAT over serial interface.
I'm thinking that once I remove the serial stuff, the whole file loading code can fit in one cog, we'll see...
Update: See post on 24Dec15 for fully functional boot from SD code.