You can't DROS just the frame of any pistol in California. That's your problem. You can't do a safe handling demonstration and it doesn't have a defined caliber, and blah blah. You can do it as a long gun, but not a handgun. Same would apply if you tried to buy just a Glock frame or a 1911 frame or whatever. The only way to get around this is via PPT. It has nothing to do with the fact that it's an AR receiver. It's a handgun, and that makes DROS on a bare frame impossible.
Another reason I've enjoyed this move to WA. AR receivers are put on your 4473 as "other." They are neither handgun nor long gun. You are welcome to build it into either.