[downinit25i]

i haven't done it myself, but I'm trying to understand your idea. If you want to reduce the vacuum right after letting off the throttle quickly then I know you can look at the Chevy AIR system. That redirected the forced air from the catalytic converter to the intake after a hard let up to prevent backfiring due to the overly rich condition created. It might not be too hard to adapt to what you want to do.

I think your basic set up would be a small valve normally closed which is sized correctly to open at the engine vacuum you've decided is correct. When it opens, have it plumbed through some kind of fixed choke or small line to the clean side of the air filter. This would lean out your mixture, since you are basically bypassing the carb, but since under these conditions you normally pull too much fuel you may end up equaling out if you design carefully. If you overdo it you may lean out too much and have to deal with those issues.