I'm using the Flask custom theme with readthedocs.org and it works. I'm also using Sphinx extensions and they work also. You can name a requirements.txt file stating the packages required to build the documentation. They will be installed with pip in a virtual environment.
readthedocs.org is a great project. What about adding it as a recommendation for extensions to host their docs?
+1 for recommending readthedocs.org in the extension dev docs. It was very easy to get up she running. In my opinion, easier than getting a package hosted on Pypi.