You are stepping into a spooky spot IMHO.
To the best of my knowledge, once a machine gun, always a machine gun. Even if it is rewelded.
There is a difference between a "parts kit" and a "demil" or more accurately a "dewat".
In the eyes of the alphabet crew a parts kit may be stripped of the receiver and built on a new receiver providing it does not violate any regulations and meets 922r compliance.
A dewat is an entirely different matter, it is in legal terms a de-activated MG and ANY alteration is in violation.
Remember the above line, once a MG, ALWAYS a MG.
As far as submitting one for approval, ain't gonna happen. Since 86 by Ronnie Reagans pen, no new MG's manufactured for civilian ownership.