I haven't but it looks like an interesting idea. My worry would be a leak that renders it useless. My thought is maybe a closed cell foam frame sheet. The problem I run into with the regular sheet is if you have a bit of a load and cinch it down with the HPG compression system, it flexes horizontally and resists flexing vertically, affecting the way the stays flex as intended.
The air beam is tougher than it sounds, not much of a worry and if it fails somehow you're not relying on it for much. The theory would be remove the frame sheet so you can narrow the pack, then air beam can be "tuned" by inflating it just enough to add back in a bit of support. It's used this way in frameless UL packs. Closed sell foam sit pads and sleeping pads are also used similarly in those packs and are popular. I don't support knock off stuff often but you can find thermarest style mats under the name nature hike for less than half the cost of the thermarest on amazon. They are good for experimenting, dog sleeping pads etc. gossamer gear also sells a few types of foam mats that are pre-cut to fold into their packs. http://gossamergear.com/nightlight-sleeping-pad-torso-length.html both would also allow you to narrow the pack, have some support and be a dual use item.