if the US treasury said that it was restructuring the USD such that there would be 21 million MegaDollars maximum, but that each of those MegaDollars could be dived up to infinity, does it suddenly become sound money?
Assuming their promise was credible (which is an insane assumption but for the sake of argument), yes! It would be as good as sound money. Divisibility makes absolutely no difference. Your 0.008 megadollars still represent a certain slice of the pie, no matter how finely you subdivide that slice.
Regardless of the valuation of each megadollar? Let's say each megadollar was equal to 1 trillion USD? Still sound?