Ooh, I wish I could answer your question, but I'm not familiar with hubzilla, and I'm still researching nodeBB. I'm not 100% sure if your question, but nodeBB does have private messaging and private group chats, but I'm not sure about the actual privacy through federation.
I'm open into looking into other options though, so if you find out more or other alternatives that would still fit into the goals/intent if the community - if be interested in seeing what you find. I'll do a bit of research into hubzilla too.
Also, I'd love it if you joined the current (temporary) forum we have right now and helped us build up the community - I'm really going to need a few solid "founding" members (for lack of a better word) to help get this community off the ground and established. I'm always open to ideas and feedback. The more people showing interest, the closer I am to pulling the trigger on setting up the permanent site through a VPS.