Is the vehicle drivable.? If it is. Is there a auto rustproofing company in your area.? They should be set up to power wash the underside. Explain what you are after, they may be willing to let you into the shop to inspect it before it is taken off the lift.
Many many years ago I worked for a rustproofing/detail shop, on occasion we would have someone who wanted what you are after. The owner was a hell of a nice guy and never turned anyone down.