In Chapman's bio of Mary, she writes that François was actually betting on just such a situation - Claude dying in childbed - but that Mary wouldn't stick around indefinitely. But, removing all of Claude's health issues from the Valois genes could be a nice touch though. Maybe she gives birth to her daughter in August, but dies in the birth, Mary's still in France because they're wrangling over her jewels/plate, and Brandon hasn't stepped in to cut the Gordian knot as it were. Henry VIII sees the opportunity and offers his sister to François and says he's willing to waive his claims to the plate/jewelry.
Now, the canon lawyers might have a hard time here. Since church law doesn't differentiate between affiliation by blood vs by marriage, which means Mary IS Claude's mother, and François' by extension. Can/will Leo X grant the dispensation? Or would Mary stand a better chance marrying a prince du sang (who? Alençon's the only other Valois left, and he's married. As is the Constable de Bourbon AND the duc de Vendôme; not sure about the duc de Montpensier though).