One big difference is in the note envelope. URB has a lot of thump and short decay. Typically a fretless is going to lack the thump and have much longer decay.
When I'm playing my big ABG unplugged in a place where a DB would normally go, I go with a very simple line and use lots of palm muting. It's big enough to thump pretty good, and the muting gets rid of the sustain.
However, getting an abg to work unplugged in ensemble requires a very large high quality instrument, and a small venue. What I understand the OP to be after can happen with a decent midprice unit.