So, I puzzled out the interface that I thought might be going on. A buddy and I were talking and he pointed me at http://bitsavers.org/pdf/xebec/104524C_S1410Man_Aug83.pdf
which I took a look at on a lark. It's a SASI manual for a SASI to MFM adapter. It has an appendix B which has a sample host interface that very nearly matches (or may actually match.. I need to see if the sample Z80 code lines up to the disassembled driver I have... It's not quite bit bang, but very close at first blush). It's not at all anything like the NCR5380 controller.... Nothing that fancy. There's 3 ports. There's read/write data. Some bits to control data vs control transactions in another register which controls the transactions on the bus. And a register to reset/resync everything. All the details are in that S1410 manual, it seems... The last page has a Z80 bus interface circuit that's plausibly similar to the chips that are on the Univation card. I'll see if I can confirm details and use kicad to draw a schematic.