there are PS2 emulator codes all over the internet for various microprocessors.

All you have to do is take one of these, and add your own matrix key-scanning codes, and itll spit out PS2 compatible ASCII.

Simple enough. PIC/AVR whatever. heck one of them is in 6805 and 6811 code .

this is EXTREMELY popular in the MAME world. something to look at.