I know a lot of fanfics break with their ‘verse to reference things in other ‘verses, but speaking only for myself, I think it’s lazy writing. I mean, it’s just as easy to say that a mask covered only the eyes and nose of a character, leaving their mouth uncovered as it is to say they wore a mask like Batman when Batman isn’t part of their ‘verse at all.
It’s really the writer’s choice, when you come down to it. When I do write fanfic, I try to stick to my ‘verse and not drag in references that wouldn’t fit. Same thing with dialogue. I try not to use expressions or slang that doesn’t fit my ‘verse. Can you imagine a character from LotR making a reference to something out of Harry Potter? It takes the reader right out of the story. I mean, would Glorfindel order butterbeer? Of course not. He’d rather have miruvor, or a good Dorwinion wine.