We're starting from the premise that you already enabled IPv6. If not so, this will help you to switch it on:

OpenWRT Attitude Adjustment 2012.09 warning: There is an ongoing discussion regarding the AICCU init script. If you can, stay with OpenWRT Backfire 2010.3.

I'm using SixxS tunnels to enable IPv6. My /etc/config/network looks like this:

config interface 'lan'
    # ...
    option ip6addr '2001:my:subnet:prefix::1/64'

config 'interface' 'wan6'
    # ...
    option 'ip6addr' '2001:my:end:point::2'

To enable that your local server is reachable via SSH (port 22) add this rule to /etc/config/firewall straight after the standard IPv6 rules:

config rule
       option 'src' 'wan6'
       option 'proto' 'tcp'
       option 'dest_port' '22'
       option 'dest_ip' '2001:your:servers:global:ipv6:address:####:####'
       option 'family' 'ipv6'
       option 'target' 'ACCEPT'
       option 'dest' 'lan' 

Also make sure that you installed the ip6tables Package, then restart the firewall with /etc/init.d/firewall restart and test your setup.