if you want to put on a mirror the IPv4 and / or IPv6 corresponding to the respective server, you may find my patch for ISPConfig 3.0.5 usefull.
How it works
Under Serverconfig is a new field "IPv6 Prefix" available. Here the IPv6 prefix of the server is entered (for a 64er subnet eg 2a01:4f8:150:2262:: ). When writing to a mirror of the vhost, then IPv6 is replaced in part by the subnet. Suppose the mirror has the subnet 2a01:4f8:150:aaaa::, then on the mirror the (master) IPv6 2a01:4f8:150:2262::1 will be changed to 2a01:4f8:150:aaaa::1.
As long under Serverconfig / Web option "Rewrite IPv6 on Mirror" is not selected, there is no rewrite, and the IPv6 is written identically to the master. So you can decide per mirror whether IPv6 is to be re-written or not (for example, failover-IP).
A new created IPv6 Is matched against the prefix, and must match. The same applies when you create a prefix - this must match the shortest IPv6 address.
For IPv4, there is no integration into the interface. It works only manually via /usr/local/ispconfig/server/lib/config.inc.local.php.
The patch is available here