Update on mice:
After spending hours and hours trying to figure this out, here's where I'm at:
Hardware: I really do think the fuses are alright, I'd recommend keeping them there for anyone trying to put this board together. Just make sure they're properly soldered, especially F1 - there's a fair amount of current running through that one.
According to spec, there should really be a nice fat bulk capacitor across 5V and GND right next to the output USB connector. This is so that power-hungry USB devices won't mess with the 5V rail on the computer. I'll put that in in the next revision of the board, but for all my testing it didn't make any difference (I used a good quality 330uF polymer cap).
Software: There has been quite a change in the USB-related files in SeaBIOS from the 2013-version my BIOS is more or less a fork of to the current one. I managed to get the newest one running, and experimented with dozens of changes to timings, doing extensive logging and trying to figure out what's going on.
My findings points to EHCI devices working perfectly (including OHCI mice connected to EHCI hubs), as well as OHCI mice connected directly that identifies as USB 2.0 compliant (bcdUSB == 0200). OHCI mice with bcdUSB == 0110 is a no-go, I get a single event (same as florian3), then nothing. No errors, just no more events. I haven't been able to debug this further, although I'm sure that could be done by someone more familiar with USB than me.
I should put in a disclaimer that all of my testing was done with a pretty limited amount of devices, so take all of this with a grain of salt.
As to why the OHCI stack seems so flaky, I'm just not sure. It might be the SeaBIOS code not being properly set up, it might be a low-level Coreboot v86ex code issue, or it just might be the chip itself not having good OHCI support. This last theory is somewhat backed up by rasteri's similar experience on other Vortex chips, using completely different BIOSes.
Bottom line, for now: some mice work just fine connected directly to the board, some require a USB 2.0 hub.