I talked to the dudes from Percy's (the guys that manufacture"speedglass") down at the PRI show a couple months ago.
"Speedglass" is polycarbonate (lexan) based. At their place in MO, they use a vacuum process to coat the polycarb with a proprietary anti-scratch coating. I've worked with several other "anti-scratch" lexans such as Hyzod and MAR10 (I think it was called MAR10) and they don't seem to hold a candle to what Percy's does.
"Speedglass" still isn't as good as regular automotive glass when it comes to scratching though. The stuff will still scratch easily, so they recommend using your wipers sparingly if at all especially when the "glass" is dirty (sand, grit, etc).
As for mounting, they do a lot of installations with regular sealant and gasketing. Riveting is just for that "coolness" factor or if you don't want to mess with sealant and expensive dealer gaskets.
As far as DOT legality, it ain't. Percy's was even making batches with a slight green tint to make it look more like automotive glass. Alas, how many of us have rigs that are perfectly DOT legal? It's one of those things that the federales shouldn't bother you about as long as you don't give them a reason to.
If the $380 bites you hard, Percy's will sell it by the sheet. Sheets go for about $350 and have enough material to cut two "wrangler" style windshields out of.
cm "lexan window fabricator extraordinaire" k