Your solution depends very much on your ISP's proxy config.
I have something similar at work, and I can only "get out" via an external dedicated https server at home to which I connect via an ssh-over-https tunneled connection.

In this case, I use this:
1.- http://bent.latency.net/bent/git/got...c/connect.html as a client in the "internal" machine. (You should compile connect.c for N900) and config everything accordingly.
2.- In my home's server I have a standard apache server (listening in port 80), and a utility called stunnel (google for it), that "converts" incoming https connections to regular http connections.
3.- Finaly, I send the HTTP message CONNECT 127.0.0.1:22 thru the tunnel, so that I can get an ssh prompt (apache server must be configured to admit proxy requests)

I don't think this suits very much your needs, but these are my 2 cents.

Regards.