I remember SAA in Johannesburg had this well and truly sorted. You arrived at the door with too much cabin baggage and there was a ground staffer there with a fistful of bagage tags who would promptly relieve you of your excess allowable and have it loaded below. Some would try it on and argue but soon realise neither they nor their bags were going anywhere until they realised who was actually in charge of the operation.
Certainly a lot of the bags being wheeled down the aisle here in Oz to be loaded into o'head lockers would not fit the test units inside the terminal.