Does your harness work? What OS supports 4 mice?
Yeah, sure it does. It works for games like Atari Football (4 trackballs) using Mame under Windows 9x (Mame Analog+), Windows XP (Mame Analog+ XP), and Linux (X-Mame with XInput).
I sorta cheated, 'cause I had my hand in getting 4 mice to work properly in the WinXP and Linux versions of Mame, which partly explains why I brought it up in the first place.
The main exception is Windows 2000. Also, I'm running Advance Mame under Linux which I think is supposed to work with 4 mice, but I'm having trouble getting it to work with one. Man - for such a complicated piece of software there's an amazingly slim amount of documentation.
Last I checked, Sprint 8 wasn't actually programmed to handle 8 mouse axes, only 4 of them, but surely that could be fixed.
'Course I zapped my power supply once while making the harness. And I've never used it for fun, only in the laboratory so far.