| Well I know it was played in studio with the Rainbow bass but, other than the last time I saw them, Les always played Tommy with his 4 string live. The main riff doesn't have any real noticable fretless glissandos that would make a fretless necessary. Honestly, Les doesn't have a distinctive voice on the fretless, so I rarely hear much difference between his fretted and fretless tracks. (Well, his intonation on early fretless stuff was a bit spotty, but other than that.)
That said, I can't think of an effect that would help with getting Claypool's tone, fretless or otherwise. It's pretty much a question of setup and gear, but mostly technique.
If you haven't already, I'd try putting on very light gauge strings, lowering your action as much as you can and playing with a fairly light touch. If you really want to approximate a fretless, your best bet may be to use pretty dead sounding string to eliminate fret noise as much as possible. Because every effect I've ever heard that tries to emulate a fretless tone has sounded very synthy and cheesy. | 
