okay - so I tried the crayon trick. As I first thought and suspected, it is a so so idea.
http://northcoastbni.com/images/ar/crayon.jpgI did it 2 ways. I just rubbed crayon into the stamp, and I heated a soddering iron and tried to melt it in. The soddering iron was absolutely a no go. It would melt up, fill in, BUT come right off like a rubber stamp with "SAFE" raised on it. it just did not adhere whether I let it sit or took off the excess ASAP.
rubbing it in, as you can see by my
quick proof of concept test in the photo, is ok - but weak. in cold weather I could see it falling out. And, it is hard to get definition of each letter. So, I say it is not a really legitimate idea now that I have tried it. Atleast, not a an aluminum body of an AR15 lower. As for it coming off and having to redone "after a while" - I would think more like "after 1 use".