Tor should help its transport proxy use a proxy, if needed.
If a censored user wants to use both a (normal) proxy and a pluggable transport proxy, Tor should psas the credentials of the (normal) proxy to the pluggable transport proxy.
The proxy chain should look like this:
Tor (Client) -> Transport Proxy (Client) -> SOCKS/HTTP Proxy -> Internet -> Transport Proxy (Server) -> Tor (Bridge)
Arturo prepared a related proposal in: https://lists.torproject.org/pipermail/tor-dev/2012-February/003318.html We should clean it up, see if anything is missing, and start implementing it in Tor.