I'd be willing to bet that possessing both would be "constructive possession" of a DD.
Agreed 100% Brian. Zero questions.
I also see the points made above and agree that others can agree reasonably that this is a way to go about it.
I also believe that a case could be made that the flare launcher is a "host" and that the insert is the "weapon" and that you might be able to register the insert as a DD or SBS and then use it in a number of different host flare launchers as you desire... think "registered sear" as analogy.