Check out some of the flying sites in Germany (http://www.segelflug.de/
karte/d_karte.html). Most of the winch sites are about 1000 meters
(appx 3000 feet). I have winched from 4 different sites 3 of which
also had areotowing going on at the same time, all were about 1000
meters. At these sites I was able to thermal away with a launch height
of between 300 and 400 meters (900-1200 feet).
You don't need an "airport" to do winch operations just any old grass
field will do. Needs to be wide enough but 100 feet would do it. How
many private grass airfields are there in the U.S. that fit this bill?
I'd bet it is a lot.

Bob