I've found a way to create triangle meshes from a PBRT Shape plugin 
without having to copy the class into your source. It's based on how the 
built-in NURBS surface works.

The key is to use the "MakeShape" function. Including "dynload.h" gives 
you access to this. A call to MakeShape looks something like

        MakeShape("trianglemesh", o2w, reverseOrientation, paramSet)

where paramSet is a ParamSet object, filled with the points, vertex 
indices, etc, to make the mesh. There is an example of how to make a 
ParamSet at the end of NURBS::Refine, found in shapes/nurbs.cpp.

There is a caveat: MakeShape returns a Reference, not a 
TriangleMesh* or even a Shape*, so you will have to pass 
References around in your code instead of pointers. Also, you 
won't be able to do anything TriangleMesh-specific with the returned 
object. However, this should still work fine for use in your own Refine 
method, which should just need to create the mesh and pass it back.