obviously, the only way to sound like an upright is to be an upright... however, i find if a play over the 15th fret (with the plucking hand, obviously), the tone is a lot woodier and softer... also take all the treble/low mids out. (-5db at 200htz helps, in my opinion).
Didn't Dearmond make some short scale bass that used various gauges of surgical tubing to get that upright "thub" direct or into an amp without having to lug around ?