Particularly, in TVTropes' "You Know, That Show" section?
I would think the polite thing to do is, do that, but if you do get an answer at B, post it at A and vice versa. Right?
You should absolutely feel free to cross-post your story-id question to as many sites as possible, with the exception of posting it elsewhere within the Stack Exchange Network.
Good form would be, if you were to find the answer elsewhere, to return and self-answer (and self-accept) so we know that you found it.
You might also want to mention in your answer (or in comments below it) where else you've posted it so that others can see what's being ruled out and so they don't end up chasing down the same false leads