Where this idea came from I forget, but in the end, I just set some black cardboard back a few millimetres behind the window as I always do. It works and meant I didn't need to hang around auto shops trying to find someone to pimp my building.
The idea didn't go away though, and since I had a spare side, rejected for wonky glazing bars, when I happened across a spray can of Humbrol Clear Smoke, I decided this would be nearly as good. Quite a bit cheaper too as the can only cost me a quid.
Paint was applied to the back of the windows and you know what? It looks rubbish.
The blackness is too close to the glass and just looks wrong. My normal method is the better one. At least I know now.