re frank,
I read:
If it isn't on your agenda now, maybe someone else has a pointer to an algorithm with which to achive that goal. Then I'd take a deeper look myself.
I think alsaplayer's sources are a good place to start looking for buffered varispeed playback(and forth ;) and there are several algos for interpolation to choose from here: http://www.musicdsp.org/archive.php?classid=5#62
HTH
x