I'm working on an ukulele of similar kind. I thought the string that I pluck with RH and the string that I push with LH don't need to be the same physical sensor. In my case I thought of short laser beams for the strings to be plucked (downside is that you don't feel them, upside is they look cool). And maybe some capacitive sensors for the strings to be pushed for the chords. In your case the bow needs something physical to touch but that doesn't need to be the same string that your fingers touch, as long as they are in line.