I definitely think it would be very helpful to distinguish between types of GMs, as there is a fair amount of difference between running an RPG event and a board game for example.
I also like the idea mentioned of having mini practice sessions: perhaps this could be implemented with a simple game (or a few) that virtually everyone would be familiar with. It could also include a short game which people may NOT already be familiar with -- to cover the concept of how to assimilate a new game enough to be able to turn around and teach it (useful for pre-releases, prototypes, etc.).
I [Cosmic Wombat Games] would certainly be willing to step up and host (or actively participate in) a seminar, workshop, or panel for training GMs how to run games in a con environment -- at least for the board game side.
Derek, as you know, we run a large number of games (and have for several years), in addition to customer engagement and demos at our booth. And that doesn't include other events such as Origins, several small conventions, and local game groups as well as numerous pitches at Trade Shows.
Before the show, I ensure my volunteers know the game, then teach them how to teach. I also have a rough (yet fairly thorough) outline I go over with them about their convention related responsibilities as well as how to handle various scenarios which may arise.
And for what it's worth, I was an IT Instructor to adult learners for 2.5 years (and am a Microsoft Certified Trainer). I know it's not quite the same, but definitely some more experience to pull from. :)
With that being said, let me know if (and how) you'd like me to help out.