If there are provisions for pre-ban/post-ban categories and if the law is written the same as the '94 ban, and if the ATF interprets the law the same, then technically the ATF considers it a pre-ban if it has been assembled into a complete, operable firearm. This includes the upper, the LPK, barrel, etc.
Technically, if they want to press the issue, they could require some documentation that the lower was assembled into a complete, operational firearm prior to the ban date. This could include receipts for the upper, LPK, lower, etc; or some other verifiable method to prove that the firearm was complete and assembled prior to the ban date.
Based on ATF interpretation, and using your scenario, the lowers w/LPKs would be considered post-ban since they had not been assembled into complete firearms. In reality, during the old ban, the assembly requirement was pretty much ignored by everyone. If the SN of the lower proved it was manufactured prior to the ban date, it was considered preban. Can't say that this would be overlooked if a new ban was implemented.