Many USB keyboards can talk PS/2 with a simple passive adapter, that (I have been told) routes the pins in a certain way so that the microcontroller on the keyboard can detect whether it should talk USB or PS/2. For instance, my Cherry G80-3000LQCDE does this (The 'C' stands for "Combo", indicating that it has this ability).
Would it be possible to add this ability to a Teensy 2.0 used as keyboard controller, by only soldering components to it? ... and giving it new firmware, of course.
Does anybody here have enough knowledge about the AVR and the passive adapters to tell?