Don’t know how to best implement this. Maybe the Displacement Mod could look at the crease information in underlying modifiers or on the Poly level before passing the data on to the renderer.
Or it could allow for manual definition of creases through a “Crease” or “Edge” subobject.
Without proper creases the use of Vray native subD’s is quite limited atm.
Would it make sense to branch Displacement and SubD’s into separate mofidiers?
You can define hard edges without using any “hard edge option”. You simply have to model it correctly, few edges for rounded surfaces and 2 or 3 -very- close edges to create a hard edge.
That’s exactly what everybody wants to avoid. It makes modeling and skinning a mess 
You can keep a clean topology with this method, with edgeloops flowing correctly. I dont see why it should be a mess as long as you keep all quads and topology friendly. I have always modelled, rigged, skinned and unwrapped this way without major problems…
Besides, I tend to collapse the mesh from time to time to clean things up. Creases and hard edges are dependant on the modifier, and once u collapse it, they get lost. And if u have to transfer your mesh to maya or xsi, or even Zbrush, u could have some problems…
I think I remember that creases on subdivision surfaces (Loop and Catmull-Clark subdivision) are patented or something…
Best regards,
Vlado
yep, that was the (official) reason for pixar sueing exluna
subdiv surface creases are patented (not that it matters over here in germany since there are no valid software patents - yet)