We just finished doing a load for one of our clients and, although not exactly a rental as discribed, this unit was brought in as a demo unit and trial and was going back to the factory after 2 weeks.
When we loaded, we were given a Form 4455 and the driver had to have that stamped by US customs. We then had to take the inward cargo manifest to the customs broker and we recieved from the broker a E29B and an A8A form that had to be stamped by Canada Customs.
We also had to have a commercial invoice showing the value of the equipment, a certificate of origin (Form 434), a declaration of importation of a vehicle for demonstartion (Schedule VII), a Certificate of Origin,
Form 4455 is a Cert of Registration, E29B is a temporary import bond, A8A is a Canada Customs Manifest.
At the end of the day, you should have a broker that really knows what they are doing with this or your client will be paying taxes on the complete unit and you, the carrier may be left holding the bag on the temporary import bond when the unit isn't returned in the timeframe on the paperwork.
Hopefully this helps and doesn't muddy the waters more...