There is going to be footage of that from the car anyway. It has a camera that would be on during the arrest. A lot of police work is already on camera as a result. It would cost a ton to monitor police behaviour, but a good system should have a way to complain about an arrest, as technology gets better I wouldn't be surprised if police officers had a small camera on their uniform.

If you complained about treatment at specific time or on a specific arrest and it could easily be looked up by an impartial person that would be great, although I believe the police officer should be given the benefit of the doubt unless he is extremely out of line as it is a high stress job often in a dangerous environment.

The increased cost of a system like this would be very high I believe. People complain all the time and I think the system would get clogged with tons of stupid complaints.