After taking a second look at the Trinket, I realized that the Arduino Tone Library wasn't supported. "Oh well", I figured, "I'll have to write something myself." Not so. Bruce Hall has a great guide on making the Trinket into a tone generator.

Another look at the card reader convinces me that the Trinket will be able to power the whole board, and provide the needed architecture to determine whether or not a card is being swiped. For the purposes of the project, the contents of the card aren't important, just that the card is present and the music ought to start.

Another update to follow with a prototype.