It is possible to forward  ipv6 ports with ip6tables to internal lxc virtual  machines.   See private-cloud

• Get an unique ipv6 private subnet: https://simpledns.plus/private-ipv6

• Add this subnet to: /etc/sysconfig/lxc-net   
  for example:

  LXC_IPV6_ADDR="fd81:2688:d2c6:5024::1" 
  LXC_IPV6_MASK="64" 
  LXC_IPV6_NETWORK="fd81:2688:d2c6:5024::/64"

  This will add an extra ipv6 to your lxc bridge.

• Add the ipv6 addresses to your lxc virtual machines on this host, find howto for your linux os.

• Add the portforwarding rules to the iptables (/etc/sysconfig/ip6tables), like:

  *nat 
  :PREROUTING ACCEPT [8:406] 
  :INPUT ACCEPT [6:302] 
  :OUTPUT ACCEPT [26:1977] 
  :POSTROUTING ACCEPT [26:1977] 
  -A PREROUTING -p tcp -i eth0 --dport 587 -j DNAT --to-destination fd81:2688:d2c6:5024::2