Oh shoot, I forgot to post my diagram.
here,
https://i.imgur.com/v6DQ5Gy.png
The idea is to extend the path of a video beam.
There's a slightly prettier diagram in an expired patent about this I have to dig up.
Since the user is looking from the beam splitter side any reflection on the beam splitter will degrade the image projected on a projection screen.
It's not that I need the beam splitter to be flexible, but to handle a bit of mechanical stress without cracking. Polymers and films are way better at that than glass.