First let me say that I do not know the actual ATF regs for a "De-Mill".
Note: to the best of my knowledge a "de-mill" involves cutting the reciver at least twice, usually done with a cutting torch.
I believe you are looking at a DEWAT "De-Activated War Trohpy". (yes I know its not really, but)
Having made a couple of "DEWAT" 1911A1 clones for holster makers, I can say that just welding plugs in the chambers would not be enough.
The idea behind the DEWAT is that it should be "impossible" to return it to a functioning weapon.
We welded barrel, pins, mag catch, mainspring housing, ect. in place. You would have to have cut it to get it apart.
Hope this helps.
PS I personally would not do this, I would make it non-functional, retaining the removed parts, and the option of rebuilding later
