If you aren't too fussy about mouse tracking quality, you can use a PS/2-to-Serial protocal converter from Vetra Systems. This way you can use any PS/2 mouse on your serial port, not just the few PS/2 mice that are bi-protocal.
http://www.vetra.com/327text.html
I personally found the tracking quality too low for my tastes. Anyone ever find another other, similar, company to make such a product?
FYI, even motherboards with PS/2 port headers don't necessarily contain working PS/2 ports. Anyone ever get the PS/2 feature working on a Shuttle HOT-433 version 1-3? Other users have mentioned that even adding the keyboard controller chip to the HOT-433 didn't enable the PS/2 mouse feature, even when using PS/2-supporting BIOS versions.
Also, not all 486 motherboards need that large keyboard microcontroller chip to have a working PS/2 port. The Biostar MB-8433UUD does without it and the PS/2 mouse port works fine.
USB mice tend to work on 486 boards with a PCI, USB card.
It would be highly irregular for a motherboard company to have support for PS/2 mice and not include the header. I'd say your chances of success here are next to 0.
Plan your life wisely, you'll be dead before you know it.