Ancestry beat us to it! That was my idea for WikiTree when we were talking about this a year or two ago. If a male descendant has taken a DNA test, automatically put a note to the effect on all the male ancestors. Then if someone else who thinks they're a descendant wants to confirm the genealogy, they can take a test and compare results. If they don't match, we know there is a mistake on the genealogy on WikiTree.
This won't be an easy functionality for us to add because it doesn't fit in the current structure, technically. We can't use categories because there would be no way to keep them properly updated for all a person's ancestors.
As for what category we can do, now ...
We could categorize users by Haplogroup, and not ancestors. That would still be interesting.