This is an idea I got from an earlier discussion on Talkbass.

Has any of the Luthier's here used a slap plate on a fretless before. Is it practical?? Is it easy to try as an after market add-on?

I know that George Furlanetto of F Basses used them on some fretless basses. Not sure if he still does, though.

Thanx for the reply I was starting to wonder if anyone is going to reply!! I hope one of the luthier's could give me some insight.

Yeah, no takers yet.
But if you want to discuss it, I've got an F w/ the plate, PM me...