Not a good idea. Imagine being inspected with a restricted rifle and no certificate. kiss your collection goodbye when they cancel your license.
You're bang on.
An assembled short barreled upper is just parts. Parts are unregulated. There's nothing saying you can't own parts.
But what would be the point of having those parts if you don't intend to use them for something? And that right there is where the problem lies. I mean, you had to have bought them for something right?
If caught in posession of a short barreled upper and a compatible NR lower, even if they're not attached to each other, the simple fact that you are in possession of both can be used to lay charges on the grounds of intent. I bet a LEO having a bad day would see it this way as well and be more than willing to let you sort it out before a judge.
Now if you DO have an RPAL you're supposed to have 30 days to register the firearm as restricted. But the onus would be on you to prove that you had the intent to register it as restricted.
Now just imagine if all you have is a regular PAL. Your chances of proving that you had no intent to install that short barreled upper on that NR lower and create a firearm that you don't even have the appropriate class of license to possess would be near impossible. How do you plan on registering it as restricted if you don't even have a restricted license?
Obviously the courts would have to prove your intent without a question of doubt. But likewise you would have to prove that you didn't have intent, without a question of doubt.
But since you've already been caught in posession of assembled components that are capable of attaching together to create an unregistered restricted firearm, I would say you have a bigger uphill battle to prove intent than the court does.
LEO's are often more than willing to throw charges at you and let the courts sort them out. Even if in the end the charges don't stick you will still have been dragged through the wringer.
In the convoluted system that we call court, who wants to be the guy that tests this out? I know I don't.