DVDdoug:
[u]Capacitive sensors[/u] might work. Or, you could use metal contacts and a grounded metal strings.
I think the strings will have to vibrate but they don't have to resonate at the generated frequency. Metal strings and an electromagnetic pickup like a guitar pickup, but a separate one for each string might work....It almost seems simpler to make an electric cello with the strings actually tuned.
There are MIDI guitars, but I'm pretty sure they are tuned normally. The MIDI pickup picks-up the note/frequency of each string in order to create a MIDI message... Then it's just a matter of programming if you want the MIDI message to be different from the string vibration.
If I mounted the strings flat on the neck just as guides, then mounted 4 of these distance laser sensors at the top really close to the surface of the neck pointing down the length of it, could they accurately detect a finger?