When needed as a backup for just the keyboard(s), simply record the keyboard-parts (thru separate subchannels) during performing live.
You can do this with either an MP3-, MD-, laptop- or Harddiskrecorder.

Me Bill:
What you say about MD is tru but I consider the MD to be not a computer that handles files, but as a recorder/playback device. There is a way to transfer songs in a digital way. (SPDIF)

I myself would never use an audio-file player on stage for accompaniment, but on Alan's question about 'keyboard backup' my choice could very well be MD- or MP3-player. (no moving parts)