Yeah... I see your point. And it's probably right.
But just to be contrary...
"Considering the inserts are not regulated now, I do not see how the ATF would allow registration of one as a DD"
The same way that thoughtful folks registered AR-15 drop in auto sears before 1986 and made a now-legal $10,000 chunk of steel out of a $29.00 one? Move it from host to host and it's the sear and not the host that is the registered item?