No, I'm afraid Fandom doesn't have such a feature. The alternatives I'd suggest, though, are:
Discussions. Of course it's not the same thing, but it still may be better than message walls. You can create a post on Discussions and ask the wanted users to participate. Mentions would also be useful in inviting the users to dive in rather than having to message them all individually to let them know of it.
Live chat group. I know that the live chat isn't a desirable option for you, but I do wonder if the live chat conversation not being limited to certain users only may be the reason why you don't want to go with it. Because in that case, there's ExtendedPrivateMessaging which allows private live group chat. Granted the future of live chat feature is uncertain as whole so it may possibly not be a long-term solution, but just throwing the suggestion out here.