I would tag it "multifandom" in the fandom field, mention any characters that showed up by name (even if they're only mentioned in passing), less because people searching for that character will want to see the fic (people searching for Mr Darcy will be muchly confused), but because seeing a list of 8 characters from 5 different fandoms lets readers know "this one is WEIRD, folks; don't expect a run-of-the-mill PWP or five things fic."

Might also tag it with "Pale Fire" in the fandom field. I'm not familiar with that, so I'm not sure if it's reasonable to copy a "pattern" from it--but "If on a winter's night a traveller" has fic that has no direct connection to the original.