They did try that very briefly. Actually I worked for a theater that tried that. I would strip down and rub real butter all over my naked body and run up and down the aisles offering real butter for everyone's popcorn. This only lasted a few minutes and I was told to leave and that I didn't even work there and some other mean things.
Smart film franchises are the best. the most expensive product film franchise apparently is Bond. BMW handle it superbly. You will never hear the name BMW being mentioned, rarely if ever see the emblem in shot. But boy do you see the hardware in action! Subliminal advertising (flash frames as mentioned previously) is illegal as under the advertising standards agency adverts have to be 'legal , decent, honest and truthfull. This is why Mars had do away with the hook 'A Mars a day helps you work, rest & play', It doesn't so they were forced to drop it.
Remember, this is the Fart Humper we're talking about. I think you may be confusing SBT and SBD again.
I was equating product placement to subliminal messaging- although it dosen't work the same way, i was suggesting that it could have the same effect