I can seem to find Serial to PS2 adapters. Probably a bit finnicky but you could solder the trackball cables to a Serial to PS2 and then connect/solder that an active PS2 to USB adapter and try it out. It also seems that there would be alot of space either at the bottom or at the top of the keyboard to hide all of those adapters/cables. The Trackball is connected to the keyboard controller right?
Internally the keyboard and ball are entirely separate devices (a Cherry MY keyboard assembly and a serial trackball module) that "share" a cable (which was effectively two cables in one sleeve). I've replaced the keyboard PCB, so now all I need to do is convert the ball and connect both to a small USB hub internally so that they can share a cable.
I'm not sure if passive serial to PS/2 devices work (unless the device was designed to be "dual mode" kind of like how some of Microsoft's USB mice also supported PS/2 when connected with an included "dumb" adapter). I might try it anyway though.
I'll figure it out one way or another. Serial must be a weirder beast than I thought considering so many terminal connectors used on keyboards have been reverse engineered and serial... hasn't?