Whoow, Tom, I am not sure I can still follow you! 
In my universe (which is partly fictional), 2D tie points are the ones you mark on an image. 3D tie points are the ones calculated by the projections. Does that help anything?
I do think that they are artificial tie points that can help convince RC that certain images go together. I wouldn’t use the term “features” because a feature, in my understanding, is only a “candidate” for a tie point. I think that it only becomes a tie point when RC matches two or more of them and uses them for the further processes.
However I do not think that they can entirely replace RCs “natural” tie points. If RC has nothing to go on in certain areas, you will not be able to force it to by adding CPs. RC will also not create any mesh in such places.