As I recall, WotC tried this before (pre-DDM). I believe that one of the two sets didn't sell very well (City of the Spider Queen) and the other was canceled because of lack of pre-orders (Return to the Temple of Elemental Evil).
While it sounds like an interesting idea, it seems to have such a low interest threshold that it isn't economically feasible to do. Think about it, the target is not just DMs that have the module. It's also only those who use minis and who would be interested in buying them in one big lot. Personally, I might fit the first two categories, but since I have plenty of DDM figures, I would only want to get those I'm missing.