I have run into this aswell. 1st time I put a plug at each end of the window and the inspector let it go. The second time (different inspector) it didnt fly. Luckly this was an eating bar and I had wall underneath. I ended up putting some plugs in just below the counter top and had holes cored in the back of the counter and with a few bushings, the inspector was now happy.
While this gave me a ok to cover it didnt make the architech very happy....Hmm are they ever
I'd prefer the 1st situation over the second. The inspector I feel has the right to use desrection while inspecting such situations. If its not an over bearing safety issue, I dont see the Code being something that is so rigid that it cant be molded to work with in more that one or 2 situations that might arise.
As for the question, I have never seen a "pop up" plug, would be kinda neat though.
Might have a look at some plug mold. I know ich but its very thin and can be placed on the window sash.