Actually no I haven't
Never have. I think the reason they haven't is because it is added weight to the tubes. Would actually weigh it down in the water more. It is a possibility, but problem is that you'll never be able to deflate it again. I do know there are companies that sell a hard tubed boat. But not sure it's a good idea. One good hit (which will happen when you run a river) and you'll damage the inner makeup of the tubes. Not sure what that would do to the floatation aspect.
I wish I could help you more on that one. But I really can't. Never heard of that. I do know there was a company that had made boats with then shells and foam injected between the plates (river boats, not thicker hulled V hulls) and they didn't last long. May be a weight issue. I do know that spray foam gets heavy when it hardens. Hmmmmm, let me see what I can find out for you. Will be a couple days, but may be able to help you out.
"Good angling ethic is not a monopoly practiced just by fly fisherman" - Bill McMillan