I had a similar idea myself. A "filter" using plain window glass. A camera type filter, even a clear one, is polished I believe. If so, it would give a similar signature to the polished objective lens I would think. I don't know if the laser would find polished optics behind a regular piece of glass or not. It would be interesting to find out. (although I wouldn't want to find out the hard way that it didn't work)
Also, the effect on sighting through an imperfect piece of glass would reduce the effective range of whatever you were shooting.
Ruck