When I was putting together a hub motor last summer I had a 36h cf i30 rim that I thought I might use but after consulting a few knowledgable wheel builders I decided against it and went with an aluminum one.
I would put more credence on going to a tubeless rim and being able to take advantage of that type of system over carbon fiber and did with my next hub build.
That would be a bad idea. With the weight of the rear hub, you need a tough but yet forgiving metal. Carbon wheels should work fine but they are not designed for hub motor specs.
You could save 1 lb or 2 by going tubeless.