John-T-53

This is different and cool. I'd go with tempered. When you think about it, many buildings have glass doors (with no frame) and glass guardrails, so why couldn't a glass tailgate work? 
 
It'll work fine. As long as you're not hauling a bed full of 6"+ river cobble, loose, it'll be fine (lol ... wouldn't want to do this with a steel tailgate either).
 
You should be able to use the bed as you'd normally would too. Put a piece of protective film on the inside - such as seismic film used on building windows - as a sacrificial scratch protectant.
 
Put a spacer under the latch bracket so it sits flush. And I think fully-framed would be best.