If I got a pre-ban upper---hoping for the sunset, and put it on a post ban lower,  just to keep in the house for home defense until the sunset, I wonder if they would still bust you if you had to use it in a self defense situation---my guess would be yes if the wrong person saw it, but how many cops both know and actually give 2 shits?

I think that most cops (in my neck of the woods, at least) will assume that any AR is illegal!  Most cops are not shooters and aren't familiar with the laws.  The rifle would certainly be confiscated, and I imagine the odds are good that someone who does know the laws would eventually see the rifle.  Then you'd be screwed.

any gun you use in a defensive situation is gonna be at least temporarily confiscated

You aren't thinking that a preban upper would
be better at home defense than a postban upper,
are you? It's your freedom, you can go about losing your rights any way you see fit. I'm not saying you shouldn't prepare for the sunset, but I certainly would NOT attach the preban upper to the postban lower. Especially if the rifle will come under the scrutiny of law enforcement because of your legal use of deadly force.

the preban police are even in your home. troll these boards too! becareful

I would not risk this at all. Get a post ban upper and put it on a post ban lower and stay legal. If you have a pre-ban upper hold it in reserve. Not to be paranoid but the reality is that if you had an illegal gun and lets say the police raided your home by mistake and found the gun you could be charged with possession of an illegal gun. Even if you have not committed a crime with it.

Like most of us, I think this pre versus post ban is really silly. It certainly has done nothing to stop crime. It is purely political. Which brings us to another point. In the case above you would be an easy target for a DA trying to pad his numbers about getting tough on crime.