Just saw this on Rondo Music's site. Not technically a lefty, but a relatively simple conversion ( think). I don't see the bridge saddle angled in any way so with a new lefty nut added, drilled bridge holes, maybe have to flip the saddles (can't tell if they're grooved or not), and your willingness to have electronics on underside of bass, it might be a decent deal for $149.95 ($300 less than a U bass):
http://www.rondomusic.com/ukb22.htmlEdit: Rondo has several models to choose from. The cheapest is a mahogany body @ $129.95. Scroll down page to see other finishes/options:
http://www.rondomusic.com/bassguitars4.htmlI also e-mailed Kurt to see how strings are mounted/secured inside bass, as I don't know if that might present a challenge for a lefty conversion or not. I'm guessing they're just knotted right under the bridge, so if you had to drill smaller holes larger the pre-existing later holes might be too large for your thinner strings and you might have to double/triple knot those strings to get them to hold. (Or you could create some smaller holed wood strip to hold them in place). Will post his response when I get it.