I just ordered a manual for my TT from Best Electronics and the schematics for the ports in the back mentioned one of the pins for the mouse port (used for the up direction of the joystick) is also used as a middle mouse button!! Actually my Mega/STE's manual is the same.
Presumably this is for the TTs UNIX clone??
I've got an idea for a hardware mod. If I take my two button Naksha mouse apart theres a space on the pcb for a third button, I then could solder a switch in to right wire and replace the mouse case shell with one from a three button PC Naksha mouse. :o)