Here's a quick update on my progress so far:

I've successfully copied the EEPROM contents from one RS15-1994 to another, so it is now possible to make backups of all your customizations (or even trade them over the web). Now that I've got the hardware part of the interface stable, I hope to spend more time trying to understand how codes (learned and upgrade) are stored in the EEPROM.

My current hardware interface uses components entirely available from Radio Shack, and costs about $6. It connects to the parallel port of a PC running Linux, and uses a kernel driver to handle the "real time" interface timing requirements. This system could be ported to Windows 95/98 (someone else has to write the driver, since I don't have any development tools for Windows). It would be harder to port to Windows NT,
as NT requires a true kernel driver (doesn't let user programs directly control the hardware). I have no idea if this would work on the Mac or other types of computers.

I've been toying with the idea of building a "black box", that would cost < $20 and interface the remote to any computer with an RS-232 serial port. The software would be much simpler (no real-time requirements), and
should be portable to any type of computer. It all depends on user feedback (assuming we ever get to the point where we can totally program the 15-1994!).