OK, I've been doing a lot of reading the past couple hours, educating myself.
There is a lot of bad information out there, and although I've seen NO DEFINITIVE LAW that specifically answers yes or not, legal or illegal to the OP's question, I've gotten a lot more understanding of what
Constructive Possession and
Constructive Intent mean.
So, I'm going to change my advice to the OP, and here is why.
IF ownership of BOTH an AR-15 rifle AND an AR-15 Pistol Upper with no lower is illegal (and remember I have not seen any definitive law stating one or the other) it doesn't matter, under constructive possession (as defined by the Cornell School of Law as
The legal possession of an object, even if it was not in a person’s direct physical control) storing it elsewhere would still be illegal as long as the OP had any access or control over it. Now it were placed in a friends safe, and the OP had no key or combination to that safe, then he has no constructive possession, and that would be legal.
As per Constructive intent (intent that is inferred to exist (as from willfulness or recklessness) in relation to an act), it really has no application here. Constructive Possession of an unregistered SBR can occur when the OP has both a short barreled upper and a stocked lower. It's still a grey area because if that stocked lower is under lawful use on a legal rifle, the prosecution would have to convince a jury that the OP possessed an unregistered SBR because he also owned a rifle, and therefore had no lawful use for the short barrel upper.
ATF has already ruled that owning an AR rifle and an AR pistol is not Constructive Possession despite the fact that the uppers could be swapped.
I do still maintain that the Constructive Possession charge is an add-on. A felon can be charged with CP if they could potentially take control of a firearm, i.e. driving a vehicle and having the means to open the trunk, in which lies a firearm. This can also apply to persons in CP of drugs and other illegal substances or items.
So err on the side of caution and do not possess a short upper and no lower for it. It could be argued by a prosecutor (how it got before a prosecutor is another matter) that you had possession of an unregistered SBR, despite your intentions to build a pistol.
Or had someone else buy it and hold onto it for me.
If someone else purchased the upper, it would be theirs to do with as they please. You can't have constructive possession of an item that does not belong to you. You can purchase the upper from them, after you have your pistol lower, and it'd be 100% legal.
Even a stripped lower should probably suffice, again provided you didn't have a spare stock laying around.
This subject comes up a lot here, and for years there hasn't been a definitive answer. Lawyers like Frank haven't been able to provide a clear and concise definition as it pertains to AR pistols and rifles and lowers and Constructive Possession. It's vague, probably by design.
You'd likely never be prosecuted for Constructive Possession of an unregistered SBR, so the risk is entirely up to you.