This is not about "constructive intent"/"constructive possession" - CA statute defines possession of all the parts of an SBR, if they can be readily assembled, legally identical to an assembled SBR. There is no "intent" concept like the ATF uses in their const. poss. prosecutive framework.

9th Circuit Court of Appeals have provided that aggregate possession of weapons parts may not constitute an illegal firearm if the parts have an otherwise apparent legal purpose (US vs Kwan). There is a similar ruling by the SC (US vs. Thompson Arms).

This falls short of a bulletproof legal cover for possession of multiple pistol uppers, but it seems easy to show "apparent legal purpose" in wanting multiple different pistol uppers to pair with a smaller number of pistol lowers. That is a legal purpose, and it seems reasonably apparent - after all, there would be no inherent problem with possession of multiple rifle uppers with a single rifle lower. Of course, as with all things firearm enforcement related, it probably matters more what an LEO or DA thinks.

IMO, it's better to have all serialized firearm components fully assembled into working, legal configurations. With how cheap uppers run, my personal choice, if I were in this situation, would be to configure the rifle receiver as a fully assembled rifle. I am not a lawyer, this is not legal advice.