Since nothing uses it now, it was decided that these could be specified using the option extension mechanism. Tony Ballardie talked about CBT border router behavior. It avoids encapsulation when transiting but causes traffic to go everywhere. When using CBT as an intra-domain protocol for BGMP, the core for G should be the border router pointing towards root of G. Michael Speer talked about distributed access control for group membership. This is done by publishing a public key in the DNS, and giving the private key to listeners.
Routers can then verify the private key against the published key when joining.
It also requires extensions to the host membership protocol to pass the keys from the hosts to the routers.