The BATF has been known to use the argument that possession of parts that can be used to create an illegal configuration constitutes INTENT to create that configuration. They have used that argument successfully in SOME prosecutions.
The biggest problem with the BATF is that they are an agency which is tasked with the dual responsibilities of both making the rules, and enforcing them. No government agency should be given enforcement power over the rules that they make. It leads to abuse of power and in the case of the BATF such abuses of power have unquestionably occurred. I wouldn't have a problem with it if the BATF made the rules and the FBI enforced them, since the division of responsibilites discourages them from making up rules (and therefore, laws, in effect) on the spot.
I'm quite sure you could maintain pre-ban parts if you had a pre-ban gun, and you can maintain post-ban parts if you have a post-ban gun, but if you have both, well, that can get kind of hairy, depending on the asshole factor of the agent inspecting.
CJ