LGTM.
I think for those who want newly accepted connections to use a "new" default stack, there is a mechanism using tcpsso that could be used to (attempt to) change the listening socket's stack so that new connections use that stack. This would need to be run for all desired listening sockets though.

We need these applied to rack_bbr_common.c as well.

I don't have a strong opinion on the default flag state (DEFAULT_OK or DEFAULT_NOT_OK), so this seems fine to me.