Changed my mind, djshox already posted a short bit of a early workflow of the process. I haven't tested or anything yet though, so if it doesn't work, don't come crying to me.
Okay so here's the general workflow for SFM working with Portal or hopefully any Source Game (maybe).
1) First, back up your TF2 beta directory somewhere safe.
2) Mystfit and I have only really messed with Portal, but we think you can get just about anything running. You gotta copy the game's content GCF and overwrite everything in the /tf directory EXCEPT for the bin folder. It is important that you leave it unaltered.
3)Rename the Program Files/Team Fortress 2/tf folder to whatever game you're trying to run, like for example I had to change mine to Program Files/Team Fortress 2/portal, EP2 would obviously be Program Files/Team Fortress 2/ep2, etc.
4) Go into the Source Binaries 2007 GCF and extract the vtex_dll.dll to your Program Files/Team Fortress 2/bin folder.
5)Adjust the target of your shortcut to match the game. Instead of
-game tf -tools -console -applaunch 440 -window -noborder -novid
portal would be
applaunch 400 -window -noborder -novid
And that's it. Just a small note, you won't be able to use gestures or voice commands. Heck half the weapon fire effects won't show up, but otherwise there it is.