Not exactly, but I roleplayed in a multifandom RPG.
I also beta read a fanfic once without knowing the canon. I spent a LOT of time on the wiki. (The person knew I was coming in as a novice, it was for a writing group. I actually really liked that he submitted fanfic to a writing group, I would've been wary of snobbery - but everyone treated it like an "original" story.)
Re: Have you ever been in a fandom that you never actually read/watched/played?
I also beta read a fanfic once without knowing the canon. I spent a LOT of time on the wiki. (The person knew I was coming in as a novice, it was for a writing group. I actually really liked that he submitted fanfic to a writing group, I would've been wary of snobbery - but everyone treated it like an "original" story.)