I've been thinking the same thing. Here's what I've came up with and this is just with a small funnel web bag. If the bag would be put on the hook it would pull the whole hook and hook link up the main line/leader on the cast due the the weight of the bag. So this is what I'm going to try. In stead of tieing the lead onto the main line/leader I'm going to use a quick link on the lead. This way I can take the lead and quick link off the main line/leader and use a baiting needle to put a bag on. Next I'll put on a silicone sleeve on the main line/leader. At this point I'll put the quick link with lead back on the loop at the end of the main/leader. Now I'll pull the silicone sleeve over the quick link and at this point pull the bag and work the quick link into the bottom of the bag. I think by working the quick link into the bottom of the bag that, that may keep the bag from sliding up the main line/leader and pushing the bottom bead up. Now if your pop-up isn't far(4-6 inches) up the main line/leader the mix in the bag will spread out close to it.
Well that's it and let me say I haven't tested this yet so not sure if it will work. But one thing that would work and would be easier is just to tie a pva bag to the lead
