Sorry for the late reply but this thread really got me thinking about a way to implement 6's in a way which would not change the game mechanics too drastically so Valve would agree to it in addition to the HL one.
I think that essentially Enigma was just trying to balance the 3 different groups which would result in a fast paced organized game CLOSEST to 6's - because Valve is never going to implement a complex system that breaks their dynamic of the game. So why not try the next best thing? And quite frankly he got it right. The 6's community should take EVERY SINGLE OPPORTUNITY to get the game out there - even if it might require some slight tweaking. It won't be perfect but why not just get it out there for people to see and experience?
I think the real trick is in setting up the right parameters for choosing classes. Valve said they're willing to add a 6v6 option - is there any way to impose the 6's structure without directly supporting it? I think the solution is to simply set "group limits."
6's is basically composed 4 OFFENSE + 1 DEFENSE + 1 SUPPORT.
http://s22.postimg.org/ql9ne4k35/Test.png
Why not work with these limits? If (very big IF) Valve is willing to set a limit of 4 offense players, 1 defense and 1 support, a mild form of 6's is already in the making. So how else can you make it like 6's? Set up another rule - maximum of 2 players per class. And lastly, allow players to offclass in their "group type" only - a medic can offclass as sniper/spy, a demo can offclass as heavy/engie, etc. (This sounds ridiculous I know but bear with me...)
There are def some downsides to this system. Obviously you'd have to include pyro. Then you might get the occasional idiot that'll go spy when the team really needs a medic or someone who will go engie when a mobile demo would be best. This would make teams frustrated but believe it or not - I think this is exactly what would "save" 6's.
After playing with these group limits, people are eventually going to realize there are optimal choices. Think of it as a training ground to 6's. People will eventually grasp that a medic is a strong general support class and there are limited times to play spy. A pyro is best for sneaky flanking. Scouts and Soldiers are favored for direct combat over a pyro. A demo can deal more dmg than an engie, etc.
Some good things - some players are really good as other classes but they don't have a permanent place in 6's. If you're a really good pyro now you can play pyro or heavy - but it's going to come at a price.
Anyway before this gets too long, it can work. It's not 6's exactly, but pretty damn close. #1 thing is that any class can play so Valve doesn't have to setup silly restrictions on not being able to play a class they created. Doesn't mess too much with their dynamic.
The only question remaining is, WHEN will the players realize that 2 scouts, 2 soldiers, 1 demo and 1 medic is going to be the best choice? Heh.
*I also liked the idea of 3 offense classes, 1 defense and 2 support - I felt this might make an interesting 6's variation but maybe it's too different...