Do not worry, I travelled from Lahore to Islamabad with 200+ kms on my Odo. The only place you maybe stopped is at the entry point of Islamabad, there's a police check post near the toll booths at Golra. Just keep your car in the right most lane and you will be fine. Even if you are stopped, give them a few hundred and you'll be fine.
But travel on an unregistered car outstation only if you are travelling for the purpose of registration. If you intend to get an Islamabad number, you can do it without taking the vehicle. There's no inspection required for cars other than corolla.
Get a UR - 017 steel plate of Islamabad and put it on your car.