Culhwch: certainly, I can't think of a time when Cat might have met Howland. Howland has not been at WF for years, and I'm assuming Cat's never seen Greywater Watch either. Possibly at her wedding to Ned?
ETA - this isn't a major problem anyway: Howland certainly knows that Ned married a Tully, so a kid who looks like a Tully claiming to be Ned's son shouldn't cause him much difficulty. And Shaggydog might provide extra credentials - who but a Stark is wandering around with a direwolf in tow?
Add to that any special abilities Howland might have, as you say, and this just doesn't seem like an issue.
At any rate, I imagine that Osha would manage these obstacles - she will have to manage far worse as Rickon's protector.