I can't say for certain, but I do know that St. Louis was initially interested in Laughton until the focus got shifted to Hayes.
We'd have more than a 6th for sending Laughton to St. Louis and we could still have moved Hayes (not that it should have ever been the matter of greatest urgency from an asset standpoint and only became a priority due to a rift with the coach).
I imagine we can still find deals that work to send a center to Boston. Laughton for whichever 1st doesn't fulfill the conditions on the Bertuzzi trade, Lysell or Poitras, and a lesser pick would be (an overpay, but) a good bit of business that gives them a talented guy to help bolster one last cup run.
Or they could make me nauseous and just trade for Couturier. Price there would probably be lower than Laughton, since they'd be taking a chance on his health, but replacing Bergeron with 90% of Bergeron is about the best they're going to do.