You are right. It's called the "fruit of the poisonous tree doctrine". In laymans terms, it states that any evidence seized in an illegal search is inadmissable in court. The illegal search being the tree, the illegally obtained evidence being the fruit. The illegal search would be the officer scoping the plates in the driveway without permission and gathering the "fruit" while checking the plates.
You could have told that cop to eat a bag of dicks and theoretically he wouldn't be able to do anything. You were not driving and he never initiated a stop. When he came up to your window and questioned you, all you would have had to say is "are you detaining me, if not am I free to go?" He would need a valid reason to keep you there and give you a sobriety test.
However, he could have pulled you over down the road for a made up reason, given you the test, and charge you with a wet and reckless (if you have those where you are). Good call on submitting to the test, as refusal earns you a trip to jail for a blood test. In my county we can be cited for a wet and reckless if we blow .01, something they call a no tolerance policy.