Thanks for the replies; they confirmed my fears so we're going to go into Lille instead. More hotel options near the station, easy car pick up the next morning...

We're undecided where we're dropping the car. Our flight back over the water is ZRH-JFK (triple AA miles) so we're debating dropping the car at Rennes and taking the train to ZRH via Strasbourg (no-change option from Rennes) v. driving all the way. Driving will be cheaper; the train will be more restful. Must decide soon.