Quoted:
If the upper is separated from the lower then yes...complete lowers with no upper can come into the state without a BB. Cause at that point, it's no longer a complete firearm.
Wait: That doesn't sound right to me. California doesn't recognize "complete" and "incomplete" firearms. All they care about is that receiver--everything else doesn't matter...including whether that receiver is assembled or not. ETA: I guess that's not true: "Featureless" rifles don't use BBs. Huh.
I'll go out on a limb and say that it depends on the FFL you're dealing with.
I've dealt with some FFLs here that had no problem handing over my internet-ordered OLLs. I've had some that would not let me pick up the lower unless I bought the bullet button from them at time of pickup, or brought in the bullet button, and the last FFL would not release the lowers to me until after he had installed the BB himself.
Best bet? Make sure the lower is off list, and then contact the FFL directly.