Let's preface this by saying I used to work in technical support for a MAJOR telecommunications company. We were all behind a firewall connection at work, and all access was done through a proxy. The proxy blocked a lot of stuff we shouldn't be using company computers to access, and then some that were more questionable. I was always trying to find ways around it.
The most effective manner was to use another proxy that I forwarded my requests through. There are a lot of them out there. If you do a search on Google for "anonymous proxies" you'll come up with alot.
The proxy logs will show you accessing the proxy server, and if the proxy server uses SSL then it'll only show access to that domain @ port 443, and hide the URL you visit. A good proxy admin will block these servers to prevent misuse.
You won't be able to do it for long if your proxy blocks by keyword rather than domain, or filters content in a similar manner by the URLs. If it does then you'd have to obfuscate the URL so that searching wouldn't pull up anything. An example of this would be
www.rewebber.de, which encodes the URL. A good proxy that does this will make the encoded URL session specific as well, so that even if your proxy admin knows the encoded URL your browser was requesting he/she couldn't cut-n-paste it back into their browser to see where the encoded URL would go, because it would be expired. These servers are hard to find and usually cost money, and would also be blocked by most good proxy admins.
Anyway, if going through the proxy is you're only way out through the firewall then streaming a connection through port 80 probably wouldn't work. I would also guess return packets would have a hard time making it back to your machine as well if you did have a tunneling program up. If the firewall allowed other types of programs through the firewall, like RealPlayer or something that DOES stream, you might be able to setup a Linux box with a tunneling program that uses those ports (7070, etc.). It would arouse suspicion with a good fireall admin though. I once got in trouble at said telecommunications company for having Yahoo! Mesenger running on my system. It made an HTTP connection every minute for 24 hours straight, had the security folks freakin' out.