I might be missing something but is that not what TreeShare does now.
If a person is in RM but not in Ancestry then there is an add button. If you do not wish to add that person to Ancestry then just ignore that and do not add them.
Then if their (deceased) parents are in RM but not in Ancestry then TreeShare will have a checkbox to add a new child to that person. Obviously do not check that.
I will grant you that the option to add that child to Ancestry will continue to appear but you could filter them by using groups and only doing a TreeShare on the group which does not have the child.
As an aside (and going in the other direction) if I see someone in Ancestry who has a child (which it knows about because that link is also in Ancestry) but who I have not previously added that child in RM then I first go to that child and add them as a new person to RM before continuing to add their parent. I find that is a better way to create that relationship in RM.