No, that's not what I mean. Actual displacement only works if you use it as mesh displacer or the inadvisable non-Raytraced object rendering (in the render tab), or changing the default render method in the object itself. By default any 'displacement' fed into a default shader or whatever displacement shader (surface shader, PF, displacement shader vector displacement...) is treated as bump in Raytraced objects. By default.
So, a separate slot for displacement wouldn't really add anything unless that would override the earlier mentioned settings. And the mesh would have to be extremely fine to get real displacement by a map that usually does the finer detail.
The thing I was thinking of is treating the different channels as vector displacement, so R would yield a different bump direction than B, or G. But maybe that's too far-fetched (if that's the right expression).