You can get stuff in a spray can that can tint tail lights - They're plastic, so I'm guessing it's the same principle... I think the trick is very fine even coats until you are happy with how dark they are.
I think window tint film would be easier to apply.
I'm buying a tinted perspex/lexan to make side windows for my T70 as well - Not to everyone's taste I suppose, but with very bright paintwork, I think It'll look pretty cool