Thinking out loud - but one workaround for now... ( I havnt tested this yet!!!)
We can create private forums, based on role, right?
So if we need a private group (and associated group forum)
We can create a private forum set to a role (eg. secret)
Make a group, and set it's privacy to not be seen. Assign role to the group users.
In the database, move the automatically attached forum from the group association. (change it's section ID)
Attache the private, role accessed forum we made earlier to the group (change it's section ID)
Now the forum posts will only be accessible to the members of that role.
Remember to delete the origional, automatically created group forum, that we detached from group.
(alternativly, one could set up the role based privacy flags on the autocreated group forum, same effect)
I realise that this is just a (somewhat clumsey) work around for now, and the fix from the team is very much needed.
Hope this helps. I will fully test this later in the week, or next. But I think this works for now.