I finished up the circuitry for both being able to actuate the play and power buttons from the computer and for detecting when the MP3 player is on. I'm sure you'll all be so amused that I used these ancient reed relays to do so — I opted against optoisolators because I wanted to have something that less could possibly go wrong — but I didn't know how they were wired. I couldn't find any info on the "Dunco Relay" MRR1CDLV8 on the Internet, and while I could identify some of the pins by a resistance check, but I wasn't sure exactly what was what. I did, however note that the patent number was printed on the cases. A couple guesses about what the smearily stamped ink said, and I came up with patent #3,575,678 … the images at the US Patent Office website entry for 3,575,678 show in much better detail how the thing works … plus it was patented in 1971.
Anyway, here's that circuit:
I figured with about 3 days before I leave, it's about time that I program the computer and hook it all up. I downloaded the software and a demo program, compiled … er … assembled it, installed it on the board, and got it running. Whew.