I've made fretless guitars the conventional way, and sustain on the unwound strings is a problem. My next project, if my brain doesn't hijack me elsewise, will be a fretless five-string baritone - I LOVE my short-scale fretless five-string high-C bass....
Five strings on a guitar too, for two reasons. One, because you can use ALL wound strings, they go down to .018" so you could hook up a set about .022 to .056 that'd work. You could do a six, from .018 to .070 or so, but those extra-fat strings just don't sound good to me. More important, to play chords on a fretless you need to be able to get two fingertips really parallel to each other, and five strings on a six-width neck would help. I'm sure Warmoth can make a baritone neck with the pegholes undrilled (better yet, they might drill the outer ones and you'd only have to drill three yourself). A five-string bridge might be a bigger problem, but still - it doesn't have to be perfectly intonate-able, just set to the right curve and adjustable for overall height. A machinist could make a SS bar that'd fit tune-o-matic posts. Many, many fretless guitarists use a sustainer by Fernandez or Sustainiac, or a Ebow. This is the mothership:
http://www.unfretted.com/loader.php?LINK=main
http://www.youtube.com/watch?v=TaCedgdyJXE&feature=related
Glass and metal fingerboards are popular. Supposedly, any auto glass shop can cut, curve and polish a glass fingerboard pretty easily.
