You could, if you could get the gel relatively taut without ripping it. Great idea, if you do a lot of these shoots and you don't mind the bulkiness of the mount.

The only reason I don't gel windows is due to the (relatively) high cost of roll media.

I think that's a fine idea Samuel. You'd have to be vigilant to keep reflections out of the gel as those would be the biggest giveaway if there was any shimmer (it's pretty tough to avoid a certain amount of slack when skinning a frame) and of course this would limit how wide you could shoot the widest shot, but it would be workable for a basic headshot.

You can purchase hard gel in panels, but it's pretty expensive. I imagine there is some sort of DIY version of this (smoked lexan).