What would be reasonable would be for them to sit out in the parking lot (and freeze their @ss off) and wait to observe a drunk patron leave and get in a car. Soon as the headlights come on, get them then. In my state, if the keys are in the ignition and you are behind the wheel, it will stand as DUI even if you haven't moved.
In my state, all you have to do is be within some proximity of a vehicle that you own and/or control.
I know somebody who hadn't actually driven in over six months (surgery problem) who was busted watching his grandkids wax his car in his own driveway. The keys were the ignition so the kids could listen to the radio. He was in a lawn chair in the front yard drinking a beer.