I don't think the auction houses ever bid something up after it hits reserve. Their game is to bid the number up to just short of reserve so that the real bidder out there in the audience who wants the car and is willing to pay the price will make another bid. That bid will hit the reserve and then, regardless as to if the bidding continues or stalls out, the car is sold, and the auction house gets its buyers and sellers premium.
The auction house's goal is to sell cars and collect premiums. Squeezing the last possible nickel and dime out of the sales price isn't the goal. If the car hits reserve and sells, they are happy! Keep in mind Mecum charges a buyers premium of 8% and a sellers premium of 8%. A car sells for 50 grand, then Mecum is pocketing 8 grand plus the listing fee the seller paid to put the car in the auction.. If it doesn't sell, they only get their $500-$1500 listing fee. Why would they risk messing up the goose that is laying golden eggs for them by trying to shill the bidding up any higher? If they did and the car ended up selling for say 60 grand instead, they get another 1600 bucks. If their shilling was to backfire on them, they have at best cost themselves 8 grand, and at worse, might be on the hook to buy the car for the selling price minus the 8% sellers premium.
Whether their shill bidding is ethical or not is debatable. It can serve the seller, if I am selling a car and my reserve is 50 grand, and there is only one bidder out there who likes the car and is willing to pay 50 grand for it, shilling by the auction house is the key that could make the deal happen. Otherwise the bidding would stall out well short of 50 grand, the reserve wasn't met and the car doesn't get sold. The seller carts his car home disappointed and the would be buyer goes home empty handed. Mecum does have a "the bid goes on" feature at the auction where after a car runs, if it doesn't meet reserve, potential buyers can go up to the bid goes on booth and make offers. So maybe a sale would still be made, but more likely not.
On the other hand, lets say I set the reserve at 50K, but I am thinking if the bidding isn't too spirited, I may drop the reserve if the bidding hits 40K. There is a bidder willing to pay 40K, but that is the max he can go as that's his credit line or his wife said she'd divorce him if he spent over what they agreed was his allowance before the auction. The auction house shills the car up to 45K, the car doesn't get sold, and both the seller and potential buyer are disappointed. In that case the shill game screwed everyone involved.