Open collectors & 74LVC245
This is just an $0.02 thought, and not necessarily an answer.

Level shifting of a bi-direction open-collector signal sometimes is done in I2C bus.  There are specialized level shifter ICs for I2C, such as PCA9306, which you could try to adopt for PS/2.  There's another method, where level shifting is done with a MOSFET connected like this



This picture shows level shifting of 2 separate lines (obviously).  More details in this app note by Philips (now NXP).