Rsync through proxy using HTTP Basic Authentication?

[Paul.L.Snyder at gsk.com]

Is it possible to rsync through a firewall that requires HTTP basic
authentication?  The RSYNC_PROXY variable seems to correctly direct the
request to go through the HTTP proxy server on the firewall, but there's no
way to specify a username/password combo.  The error message reported by
rsync is "bad response from proxy - HTTP/1.1 401 Authentication required",
which is not unexpected, under the circumstances.

If I'm reading the manual correctly, the USER and RSYNC_PASSWD environment
variables only affect rsync's authentication to the remote rsync server.

With many apps, you'd specify http_proxy_user and http_proxy_pass in your
environment, but this doesn't seem to work with rsync.  (The fact that it's
not mentioned in the manual made this a thin hope, anyway).

If there's absolutely no way to do this using rsync alone, does anyone know
of a simple HTTP proxy program that could be configured to perform the
authentication on its own?  That is to say: one could run this proxy on the
local machine, configured to listen to only local requests.  RSYNC_PROXY
would then be pointed to this local proxy.  When an rsync is initiated, the
local proxy would pass the request to the firewall, and respond to the
firewall's 401 with the correct user/password combo, with rsync none the
wiser.

Any ideas will be gratefully received.

Thanks,
Paul