You could probably get away with a single install of bbPress, but you would need a plugin or some theme work.
After installing and integrating bbPress with WordPress MU, I installed BuddyPress and got the group forums working. I created a group and saw that the forum created for that group was a child of the forum that had been created during the install of bbPress. I changed that forum to be a category, so that no topics could be posted in it.
It is entirely possible that you could filter out content that belongs to that category for the main forums. So when using BuddyPress groups, you can see group forums, but when viewing the bbPress forums, the BuddyPress forums are filtered out.
I am also using the hidden and read-only plugins with the BuddyPress Groups for bbPress plugin, and that is working well. However, you could suppress the content rather than marking it up.
