WELL. I finally got it licked. It was simple enough but I'm not sure my coach will approve because he's been totally against me putting a stronger bit in Monty's mouth. Um. Anyway.
I stuck him in a mullen kimblewick. I'm not using the slots at all so there's not much leverage, if any - its action is basically that of a hanging cheek snaffle with a chain. VERY mild curb and poll pressure. He'll work in it on the flat exactly the same as he works in his French link snaffle (but it's not dressage legal so we're sticking with the snaffle for our dressage), it's when we're jumping that the change comes.
He still gets real excited and forward but the kimblewick combined with a few well-timed half halts seems to have totally solved the problem. We're cantering confidently into our fences (I haven't done that since my little grey Welshie this time last year!) in a controlled manner and we can even do related lines without him turning into an idiot.
The biggest change is in me. I am SO much more confident because I know I have more control, and that changes my riding for the better. I think he's responding to that more than the change of bit.
I'm so excited. 2012 is going to be one heck of a year.
Now off I go to finish the last of these jump standards...