So the wiki and various youtube videos indicate there are two other racers: Gasgano and Ben Quadinaros.

However in attempting to emulate with Demul Ive been unable to determine how to utilize them. If anyone figures it out or knows the answer, please comment here!

According to www.segaarcade.com and a very old gamefaqs guide, Gasgano is released after the machine has registered 700 games (?!) and Ben becomes available after the Boonta Classic course has been cleared 50 times.

I will be attempting to do this (atleast with Ben) to verify. Demul began experiencing an Error 7 that required me to clear the swracer sram and eeprom to get it working again, which would clear any of the wins Id recorded previously. If it works I will release an unlocked eeprom and sram file in the Files section.

On the machine we have in the arcade all 4 racers are unlocked. I have really only played with Anakin and Sebulba though. If there is anything you want to know about the other racers I can try to find that out next time I'm there (I usually go once a week).