It's not 3rd-party - it's part of IOKit. Look at the code MacMAME uses for reading joysticks (macinput.c). You'll have to hack it up to read mouse input the same way. I'm still on Panther, so I can't help with the latest MacMAME (I'm also very busy right now, so I'd rather not commit myself to it).
If you do change it to use HID manager, mouse input will all be perfectly linear - the acceleration is done in the driver. Also, Wacom tablets won't work at all.