Well, there are software opensource libraries for DAAP and also some software players support it. Mostly for linux though. For windumb only GIT (rudimentary java player thing) does (unless you count commercial stuff).
So in principle there wouldnt be a problem – if there WERE any hackable hardware player. I’m simply not aware of any hardware soundplayer where one can freely replace the firmware with a linux clone. If there were it would be doable. So find a way to built a soundcard into a NSLU, then its no problem…