I've been able to get through Security for a defferent terminal than my flight by showing a PriorityPass and telling them what lounge I'm using. I don't do that too often- since I never know what type of mess I'll encounter when re-entering landside for my flight, I end up leaving the lounge way too early.

And, Lurker, I don't think they'd get picky about how you got in if you needed to consult the attendants about flights. If you're flying Airline A and you're in Airline A's lounge, they should be able to help (although they may be nicer to a top-tier Elite). It's just that they won't be much help with flights on Airline B.