Years ago, I had a gimmicky device called, "The Fretless Wizard" that simulated the sound of a fretless on a fretted bass. (I still have it lying around somewhere in storage). The principal consisted of a frame of thin tin? saddles that suspended above your bridge, which you lowered down to just :kiss" the top of each string via small thumbscrews. It also had a lever that you flipped up which allowed you to disengage the unit when you wanted to go back to fretted sounds. My recollection was that it sounded pretty convincing, though it was a somewhat cumbersome contraption. The same guy also made one for guitar called, "The Raga Master" which gave your guitar an electric sitar effect using the same principle.
Well, enter the Di Piazza bridge:
http://www.youtube.com/watch?v=C8MEIR0IRoAThe sound is all in the bridge designed by Mike Sabre/Mr. Di Piazza, and the good news is lefties are available for both 4 and 5 strings! Yeah, it's pricey, but I think you'll agree it sounds pretty amazing. Supposedly fits most stock P and J style basses. I'm not sure if the fretless sound can be disengaged, however:
http://www.mike-sabre.com/dipiazza/dipiazza.html