A lot of sound equipment will let you tap in the rhythm to modify the tempo of a playing CD, for example. The problem is that you want to do that hands-free, but beat detection only works on hard constant beats, not on violin sounds. I doubt there's any free software that will do playback time-stretching properly in real-time, leave alone the kind of beat detection that you're talking about.

I would invest in a hardware unit like a mini-jam unit made by Boss which allows tap rhythm adjustments and then look for a pedal controller which you can tap on with your foot, but obviously the input would have to be mapped in MIDI to allow you to interface the pedal. Alternately a software setup with Cakewalk Studio or something similar might also work, but the usability will get more little complex.