Roundwound wear on fretless fingerboards is highly exaggerated. You can always use tapewound (I dislike them) or a coated round. GHS Preassurewounds would also be a good choice. But you gotta have rounds of some kind for growl. Flats have purcussive attack/punch, but they don't growl.
Geddy Lee uses Rotos on his fretless Jaco tribute bass which makes it sound oh so sexy on the only tour he ever used it on and for only one song...

