I don't think you would have any trouble with that. An unexpired temp registration is still a valid registration.
In my experience, if you're honest and straightforward and cooperative, the border officials are not out to make your day a bad day. Yes, they have the authority to detain you while they search your vehicle etc., which may happen due to random chance or not having crossed before or whatever, but keeping a cool head and a good attitude and letting them do their job will generally get you going before long. I've never encountered officials who have been anything but professional; sometimes humorless or even slightly brusque, but professional.
