OK, update time: Got the mouse working by adding an ISA serial/FDD/Parallel/HDD controller to the ISA slot. Oh, man, did that create problems. Firstly, mouse started working! Thinking everything was good, I assembled into an ATX case. No dice, that case didn't have a FDD slot. OK, pack up new, previously-unopened case and put downstairs in the heap.
Next, disassemble 1.4Ghz Tualatin and 8500 combo and install! No dice, case not wide enough to take AT mobo.
Last! Use last of three AT cases! Fits! Gotek with FF doesn't work. Can't figure it out. Couple hours spent working with FF.cfg and cables. Problem was:
ISA serial card had a FDD controller. Plugged Gotek into that, disabled integrated MOBO controller and works!
Currently FTPing via PCI network card over my games.
I think it's working.