Ainda não tenho uma resposta definitiva, mas entrei em contato com o CBP e parece que eles não têm uma política oficial sobre isso. Os requisitos parecem estar ao critério do centro de inscrição específico.
Aqui está sua primeira resposta:
You can log into your account and register any vehicle you wish.
If you register your vehicle in SENTRI as part of your initial application, there is no fee for registering your vehicle. If you wish to register another vehicle, you must pay a $42 vehicle registration fee. There is no longer a requirement to visit an Enrollment Center for a vehicle inspection when registering a new vehicle.
You can register up to 4 vehicles for use in a SENTRI lane, and you can have up 8 SENTRI members registered to a vehicle. Approved SENTRI members are able to use the Global Entry kiosks when re-entering the U.S. by air and TSA Precheck, as long as they meet the citizenship requirements for Global Entry and their fingerprints and passports are on file. They are not, however, entitled to any benefits under the NEXUS program through their SENTRI membership.
Isso parece ser uma resposta genérica para mim, e embora eles mencionem "qualquer veículo que você queira" na primeira linha, eles falam sobre "seu veículo" ao invés de qualquer veículo, então eu perguntei eles para esclarecer.
A resposta deles ao meu pedido de esclarecimento:
You will need to call the enrollment center and ask the officers for those requirements.
Sometimes individuals have said they were told they could register vehicles with only a letter from an owner and others have stated they were told the owner had to be present when they registered the vehicle.
Vou incluir uma atualização assim que receber uma notificação de um centro de inscrição ou dois.