I’d suggest getting this to work in pure VTK first with a simple example and then you can adapt it to work in Slicer.
Did you look at this class?
http://www.vtk.org/doc/nightly/html/classvtkProjectedTexture.html
You can also have a look at how vtk8 with the OpenGL2 back end exposes render passes.
It seems that to do projected textures correctly you probably need to use some shader code - there’s some nice detail here: