Hi,
Looks like "show bgp ipv6 summary" is available on ERX devices only [JunosE]. I don't see it on an MX.
Also, it is possible to exchange inet6 routes over v4 peers, so I guess its the table inet6.0 under the BGP peer that is more important? It seems to be along the same logic that there is no "show ip route" or "show ipv6 route" in Junos but rather "show route table inet.0" and "show route table inet6.0".
Interesting to see if such a command is introduced in the future. I feel this may be needed especially when you want to list ipv6 neighbors only.
Cheers,
Ashvin