I don't supply any technical documentation, but I think KRIKzz plans to. There is section in manual about home brew, but it doesn't get technical:
Homebrew Development
USB Port allows the user to load the ROM file from a PC to cart memory and then run it. This is much quicker and easier than transferring to SD card for testing and debugging.
The USB port also allows the transfer of data back and forth between the PC and homebrew software.
Homebrew software can load its own customized firmware into the FPGA.