I think before you try this you should check on the laws in your state.
The sheetmetal replacing the glass would not necessarily be a bad thing in a crash
because the metal will absorb energy from the crash in its deformation process.
However, I think you will run afoul of the same laws that govern the density of tint
on the backlight (that's the rear glass). I think you might circumvent this by installing a full time rear camera and monitor.