So I bought a really old PS/2->USB adaptor. A "PI Engineering Y-Mouse" device, which promises the ability to connect up to two keyboards or mice to a single USB port. Apparently, they still sell a newer model at $60. The one I got was in a sale of unwanted equipment, probably from when USB first came out and didn't take over the world, and cost quite a bit less.
I hung it off my spare M, leaving my main one connected via PS/2, and it seems to work- amusingly, the lock leys on each board affect both.
Except for Scroll Lock on the adapter-connected board. It doesn't register or set the light that I can see, although the one on the PS/2 connected board will illuminate both boards' LED.
Is this typical of USB adaptors?
I sort of like... just for the sheer obscenity... the concept of using the adaptor to hook two full-size PS/2 Ms up side by side.