Answering my own post...
Source for the HxC file selector is at
https://github.com/jfdelnero/HXCFE_file_selector
From briefly looking at that, it seems the Amiga can read/write any sector on the USB drive. That means, it would (in theory) be possible to write an Exec .device which in conjunction with fat95 would allow full access to files on the USB stick!