If you could use a little laptop/netbook as an MP3 player, one idea is to take apart a USB Mouse. You could just take the wires for the short connection, piggy them up to the left mouse click button and probably blind out the sensor/take out the ball whatever as well.

Use the trackpad on the laptop to hover over the 'play' button in your MP3 player software and your short out connection should hit the button for you. Hopefully.