Il'l bet the 21fb glass probably was laminated, Im not sure. SInce 21fb's came with no safety glass, it probably was up to the set manufacturer to come up with the safety glass. They probably weren't all the same.
I used to have a spare 21fb glass, and it had a big black rubber gasket around it. There would have been a space between the glass and the tube. I don't think there was tape. If something like this took an impact, it would be less likely to break imho because the gasket would absorb some shock. Also, the impact force would be distributed around the edge of the tube where it is stronger. I'll bet the second part of that is more important.
Even if the safety glass is just glass, It doesn't have the force of vacuum against it, and would likely be stronger than the screen.
On a 21fj, I think both layers would have been expected to break, and the broken glass would probably hang there on the pva goo like a broken car windshield.
I'll bet it would be a real pain to cram lexan in there because of the curve of the screen. You would also have to seal the edge. Maybe you could find some original 21fb glass & mounting hardware. Now im rambling...
P.S, thats a great looking set.
John