There is a company making a clear AR-15 lower now, at least partially for teaching purposes. It's supposed to be sturdy enough to take a good bit of abuse, although it wouldn't be recommended for rough duty.
Here is a video of them shooting it showing the trigger pull, hammer reset, and trigger reset. I just wish they had a slow motion version as well.
https://www.youtube.com/watch?v=ihSOPiHOgMY#t=56
I could see this being a useful tool for instruction.