Is this just hypothetical or is there actually a drive-in nearby? Can't picture any myself, if there were any in the area I'd guess they'd be a good 40 miles out of the city.
They would probably let you in as long as you pay admission, but I don't think there are any close by. For a similar (sort of) expieriance you could ride to one of the movies in the park throughout the city.
It would definitely be fun! As others said, you'd likely be able to get in fine (a lot of these places charge by the car and then additionally for each person). Also, you probably wouldn't be able to get audio unless you had a FM radio handy. The old drive-ins had speaker units you'd drive up to for sound, but most of them switched over to simply broadcasting the audio over an FM frequency so you can tune in through your car stereo. Nevertheless, it'd be a fun trip...be sure to let us know if you put something together
Every month during the summer in MIlwaukee, The Bicycle Federation of Wisconsin sponsors a Bike-In Movie near the Holton Ave. Marsupial Bridge in the Media Garden at Water and Pearson Sts. on Milwaukee's East Side. The Trocadero is across Water St. from the garden. It's usually every third Friday during the summer.