That makes sense, the province of Quebec lists a tax rate of 14.975% on vehicle registrations, so they must have to establish some sort of value. There is also a requirement for a safety inspection number or certificate number (the exact wording varies by province). There's no info online on how a vehicle's value is established though. In California, for example, they ask what you paid for a used vehicle in order to assess a sales tax. If it sounds reasonable, they go with whatever you tell them. If not, they'll ask for a third party's (dealer, mechanic) written estimate.
It seems like this could be as simple as going to any mechanic with a few pictures of the vehicle and having him write up a vehicle value estimate, detailing required work and level of rust/corrosion. A few extra dollars to the mechanic for a low estimate will be recouped during the registration process since 15% is a pretty heavy tax rate.