"It would look really bad if she was caught again without underpants, especially when she has a top lingerie label backing her, so they'll want to ensure that doesn't happen."

I disagree. Just make that part of the campaign. In fact, I would make that the ENTIRE campaign. Scenario: she normally hates underwear, but in their case it's so beautiful and comfy that she'll make an exception! It's a variation on "I wear _____ - or I wear nothing at all."