limit adaptive subdivision to polygons with displacement material assigned

I often get sent really large landscapes with river beds and other various areas that need adaptive subdivision to make the displacement work.

The problem is that the entire landscape has to have adaptive subdivision enabled, which leads to massive memory usage and calculation times.

I can separate the meshes, but then I lose the smooth transition between the meshes when they non-adaptively subdivide at render time. I suppose I could collapse the subdivision and then seperate, but that would lead to poor viewport performance when compared to subdividing at render time only.

A solution would be a checkbox in the subdivide modifier, which when checked signifies to Cycles that the adaptive subdivision should only affect the faces of a mesh which have a material with 'true' displacement enabled and a connection into the suface's displacement input.

Alternatively, the user could control the adaptive subdivision with a vertex group.....the weight could control the dicing scale.

Ideally, both of the above :)