It wouldn't be hard to set up a pretty innocuous looking system whereby you could mud from work, assuming they give you normal web access and don't have a keylogger installed on your machine.

1. Get a generic sounding domain name and map it to a static IP or one of those services that lets you automatically update the DNS entry for your domain. This will point to your home machine.

2. Set up a web server with a fake storefront or some other "real looking" content that doesn't identify you. This way an admin won't be disappointed if he loads the site in a browser to see whether it's legit. Also, when you register the domain name, use an anonymizer instead of your real info in case they do a whois lookup.

3. Set up a ssh server listening on port 443. It's unlikely this port will be blocked (or monitored) since it's used for SSL. SSL traffic is encrypted, so the fact that you're sending non-plain-text data to port 443 shouldn't raise any red flags.

4. SSH to yourdomain.com:443 and use the port-forwarding feature to foward localhost:9999 to carrionfields.com:9999. The three main Windows SSH clients each support this: PuTTY, SecureCRT and the OpenSSH/Cygwin port. The latter is simplest, imho.

5. Load up your favorite mud client and point it at localhost:9999. Your packets will be encrypted and travel to port 443 on yourdomain.com, at which point they'll be decrypted and forwarded to carrionfields.com:9999.

The main flaw in this scheme is that if someone were to actually telnet (or ssh) to yourdomain.com:443 they would immediately know you're running a ssh daemon on that port. That would raise huge red flags, since it would be pretty obvious that you're trying to cover your tracks.

Also, if your employer does periodic disk scans to detect third-party software, use a client like WinTin or yTin that doesn't have to be "installed" in the Windows sense. Compress the client and your scripts into a password-protected zip file, then rename that file to something drab like "temp.txt". When you're ready to play, just rename the file and extract its contents.