The wording of polymorph doesn't have anything related to this, at least not in the PHB. I'm assuming that the wording is the same on spell cards.
What I'm considering doing is an opposed strength check, and if the player succeeds, he will burst out of the boss Alien-style, causing him to take probably bludgeoning damage, and then some bleeding damage at the bottom of every round. If the boss succeeds, the player will be regurgitated and take bludgeoning damage, and will also be prone. Note that this hasn't happened yet, my players are just trying to figure out how to pull unnecessary shenanigans.