Yeah, almost a decade ago some folks tried it and posted their results all over the Interwebz. Consensus was that it was really really hard to ride. Edit: unless you’re Roger Davies.
http://www.myspace.com/video/uniracer/coaster-uni/319841
http://www.unicyclist.com/forums/showthread.php?s=&threadid=22729
im not sure if this question has already been asked, but the other day me and a mate who rides a trials 20" bike were thinking, i wonder if they could put a freewheel on a uni, just like bikes do, so that u could pedal to get urself going, then just stop pedalling and u would keep going.
you would have to do something with the hub connecting to the cranks, but i think that should be the next prototype uni, they’d be a hoot to ride