I have used a scope on a caplock, and yes, it was fouled. I wrapped the scope in a sleeve made of cloth, but it didn't help much. However, the lenses were not affected, as they were out of the way. It was the scope body that was contaminated.

My suspicion is that a pistol scope may not work for you that close to the eye. Most that I have worked with have pretty lengthy eye relief. You may have to use a rifle scope, or move the pistol scope further forward. Good luck with it, it sounds like an interesting project.

Take care, Tom