It’s not that hard to rip off spirit-gummed moustaches/beards. It usually takes a bit of skin off with it and that smarts a lot. When my high school did South Pacific, most of the sailors were actually girls, so almost everybody was wearing some sort of fake facial hair. The one guy who was wearing a fake moustache decided that he was too impatient to wait for the remover, so every night he just ripped his moustache off. I don’t know if he just had really tough skin or if it was just that he actually shaved his upper lip and we girls didn’t, but four year later, I still can’t believe that he did that eight times in five days. I tried, and I ended up taking off a chunk of skin off my lip.
If you were going to try to glue something with any weight to it, then I’d wager that spirit gum isn’t strong enough. Anything stronger and I think you’d be looking at a pretty bad scab or worse afterwards.
In the movies and in theater, when it looks like an actor has metal glued to their skin, it’s usually actually a lightweight rubber or plastic prosthetic or mould, and they’re using spirit gum or a similar substance to attach it. It might be a very lightweight metal, but probably not.