It would be cool ...
... but can't recall what publication it was in ...
Byte, Dr. Dobbs, Computer Notes ???
In the UK, there was a (2 part) article in 'Electronics, the Maplin
Magazine' on doing this. But you had to byy the firmware ROM
pre-programmed (no source or hex dump in the article), and I suspect it's
not impossible to find anyone who has one.
-tony