David, it is NOT advisable to think of using the mover as some kind of wheelclamp.
Firstly the pressure applied to the tyres over a long period of time could well deform the tyre.
Secondly, the continued pressure on the mover may have detrimental effects on the rollers and operating gear.
Thirdly, it is very easy for the determined thief to simply slacken the mounting bolts and slide the rollers away from the wheels.
Lastly, there is no way that a mover would be classed as any kind of anti theft device, as it is not one, as even with the rollers engaged, a determined thief would simply pull the van with the rollers engaged, which would terminally damage the motors.
Hope this helps.