Just realised the Layer is only on the outside surface of the glass so it wouldn’t interact with internal bounces.
Maybe neg (or pos?) assymetry SSS in the Layer, would mimic light scattering outward (or inward) as it leaves the glass, a seperate but likely phenoma?
Somehow I doubt the effect is caused by refracted rays leaving the glass.
I think the optimum code would be something that scatters light along the normal angle to the internal ray bounce.
Edited because I forgot Bella terminolgoy.. derp.