As per federal law the receiver is not a rifle or a pistol, but a firearm. Per Hawaii, as stated by London, it can only be one or the other. If that is correct and legal, don't know. I think I heard that the FFL that you receive the lower from can specify if it is a pistol or a rifle lower before you register it.... Something like that. Try talking to a FFL about that.
Sorry, I have never wanted to build a AR pistol, and never researched it, just trying to help with what I remember off of the forum and hanging around the LGS.