Pharaoh,

On a divert, usually it's been my experience that operations/dispatch would rather not have the passengers deplane unless there is going to be an excessive wait. This is because they often scatter to the winds, and then there's the strong potential to be delayed even further.

I've had two experiences with your question. The first one was where we remote parked (near an FBO) and a passenger asked to deplane since the divert field was closer to their final destination. We tried to coordinate it with dispatch, but the powers on high denied the passengers request since we were on an unsecured ramp and locating the passengers checked bags was not possible.

However if we had been able to go to the gate, and the person didn't have checked luggage, I would have been hard pressed to find any person onboard that wanted to leave the flight (permanently), if that is what they wanted. However it would be worth prefacing the statement that doing so may have repercussions on any other tickets attached to that reservation. (Say for instance the flight was Washington to Chicago and diverted into Indianapolis, and the passenger wanted to rent a car to drive to Chicago to catch a connecting flight. The connecting flight reservation might have issues.)