Danny Ferrington built an acoustic bass with wooden frets (ebony) for Don Was.
there's a track featuring it on the CD with the Ferrington guitars book- a woody

quasi-fretless sound.
British company Bond built a guitar with a graphite neck and fingerboard, featuring steps moulded into the board to act as frets.
but I think most non-metallic fret materials wouldn't last long with a set of Roto swing bass rounds grinding into them.