Let's say your unregistered car was sitting on your property and it got smashed into, you can definitely claim. But hmm if your car is unregistered and it was parked on the road when it shouldnt have, i guess that the other insurance company can say that it should not have been on the road in the first place !
Hard call bro, try make your car look discreet ok ! Take of the rims etc is a good idea.