I think this is going to hard against the "economy of gift" and isn't going to play well in the end. If they were hosting their own forum / mailing list, charging to access the community would make sense. But the forum is hosted by a company that gives it away for free. The people posting are posting freely (and may not be associated with the project). Some of the people posting answers are members of the project, but some are not. If the maintainers get an answer from someone else are they obligated to pay the answerer a maintenance fee?
I would limit this to "if you find yourself asking about an issue or posting an issue", since those are points where you are looking for help not just from the community at large, but from the maintainers in particular.