We have block-level access to the entire card, regardless of its filesystem (see bootflags.c, using Canon routines), and there were also attempts to access the SD and CF card directly (recovery branch for SD, cf_acc.mo or hdparm branch for CF). There is even a custom filesystem driver (fullfat) that runs in bootloader (recovery branch). In QEMU we have both SD and CF working, so the low-level side is documented there (SDIO/CFATA, SDDMA/CFDMA). There's also
this UHS hack, which might be helpful (at least as background info).
I'm not sure how the high-level side works, but g3gg0 appears to have figured it out in io_crypt, so I'd say a filesystem driver is totally doable. However, I don't see it as a priority.
In other words, if want to port this thing, there shouldn't be any major roadblocks, other than time and manpower. I can help with advice on the direct access interfaces for SD/CF and how to debug in QEMU.
