Rendertime Displacement for Position Displace Op

[AndyMurdock]

I use Position Displace Op a lot. It works well for a displacement with a uvw mapped object, but what about when the displacement map is more complicated. 
I also use a great deal of VRay triplaner mapped displacement and the VRay Blend Material that mix different displacement maps. There we have displacement only happening at render time. 
I'm hoping for a way to find that render time displacement and give it the Position Displace Op. 

Thanks 
Andy