SUCCESS AT LAST!!!!!
Jumpering in the cable to set ready didn't seem to work, but putting a jumper onto J0 between pins 21 & 22 solved it! I'm now reliably reading and writing to the HxC as an internal drive. I'll check out configuring the HxC as DS0 and DS1 internally sometime tomorrow, but at this point I've got a fully working system with a reliable long term storage system!
Thanks Chuck and pski! Maybe this week I'll get some time and be able to take some photos of the whole power supply, FDC connections, etc.