I’m no lawyer either, but I agree with PrintDriver that this sounds like a parody situation which could arguably be a defense against a copyright infringement claim.
From what you’ve mentioned, the parody poster doesn’t sound like it’s meant to make money and will only be used in-house as a joke of sorts, which could arguably be another point in favor of it being fair-use situation.
If a problem arose, though, you’d likely get dragged into it, but the chances of a problem are probably slim to none.
I have a couple of clauses I put into client contracts when they seem needed. The first is a warranty, of sorts, from the client stating that all materials provided to me by the client do not infringe on a third party’s intellectual property rights.
The other clause is an obligation for the client to defend and indemnify me against any third-party claims pertaining to misappropriation or intellectual property infringement.
I can’t remember the exact wording, but it came from an attorney friend who suggested this as a way to protect myself from foolish clients who supply me with resources, like photos, that they don’t have a legal right to use.