Reading through Barefoot Ken Bob's website the other day, I did find a couple of things I missed before. First, though Ken runs barefoot on pavement, he states , "I actually prefer that beginning barefoot runners start on rough, rocky, uncomfortable terrain."
Second, his first barefoot marathon was actually a trail marathon. After that, he began running marathons barefoot on pavement.
Overall, it looks like there is a good precedent here for learning to run barefoot on trails first, before running barefoot on pavement.