Follow the sourceforge link and browse the CVS...
http://kbdbabel.cvs.sourceforge.net/kbdbabel/kbdbabel/kbdbabel-mac-ps2/
The hardware looks pretty easy to build by hand. It uses a Atmel AT89C2051 which uses the 8051 instruction set and is in circuit programmable.
So, not nearly as hard as it could be.
Nathan