The document you sign to indicate you understand the obligations involved starts out with a paragraph on this subject. You can use any vechicle when driving through the nexus lane, including a rental vehicle. The licence plate must be visible and unobstructed and all passengers must be Nexus card holders. I had already asked the agent this before I saw the document, and she said that a rental car is fine. Also, if I choose to buy a car at some point, I just have to pop into their office and provide them with the vehicle information. I assume this isn't mandatory however based on the document I signed.