What are you using to convert your 3D meshes to point clouds?
Just thinking out loud but if it was possible to convert your mesh into a particle system it would open up more possibilities. For example each particle could be replaced with a camera facing 2D shape that can have a material applied. You can do all sorts of things with that.
As is stands I'm unaware of a way to have Arnold render the individual points in a point cloud as a different shape but someone better versed in Arnold than me may know of a way.
Ben Bisares