As an IT guy thats been on the "other" side of the fence....it's their network, their connection, their hardware. They get to set the rules. Probably part of your employment contract or company policy manual, too, if they are current with the times. If they don't want you on, don't try and get around it. Their time, their toys.
That said....
Have you tried asking? I have allowed access to forums when folks have asked...as long people ask nice, and they are sure to say that it's for access on break time and lunch, etc...and that they know that access times and usage show up in the logs.
I've also been on the receiving end...asked for access, got it.
I am having a time right now at one client, with folks listening to streaming radio. That's killing the Internet connection for the people that really need it (lots of FTP transfers in and out there). Block it, and they find a way around, or a new site to listen to. A simple google search finds ways around or secondary access. Boss has laid down the law, several times...he's picking who to 'make an example of' now... The one I think is gonna get cut listens to a LOCAL radio station over the Internet...could do that with a ten dollar radio.