This is what ATF calls "constructive possession". It has been applied in the case of machineguns; for example, if you were caught with BOTH an AR15 semi-auto rifle AND an unregistered auto-sear, you would be guilty of possessing an unregistered machinegun, even if you never put them together to make an actual working machinegun.
I'm not sure if constructive posession is so clearly applicable in the case of the 1994 Assault Weapon law. I would imagine that, if you owned both pre-ban and post-ban lowers, it would be a hard sell for the ATF to convince a jury that you intended to create an illegal assault weapon. Perhaps you might be in a more precarious position if you posessed pre-ban uppers but did not own any pre-ban lowers, but your particular case sounds pretty safe.
Has anyone out there ever heard of a constructive posession prosecution in connection with the AWB ?