Just to note some closure on this: it turns out the Bonvoy Customer Care rep (contacted through their link from the Facebook Bonvoy InsIders group) came through, The Delta hotel is now going to let me park my car at the hotel for one day beyond my checkout day. It seems this is being allowed owing to my Titanium Elite status along with that held by my friend who has a reservation for another room at the same time. Therefore anyone who is considering doing the same, i.e., leaving their vehicle beyond the check out date, as some taking short trips from the area on Amtrak might want to do, should note as a general rule the Delta Hotel does not allow this. But it makes my life easier that in this one instance they are allowing it.