As a better workaround I created a new SST plugin - called it xtrabackup-v2-ipv6. I merely copied /usr/bin/wsrep_sst_xtrabackup-v2 to /usr/bin/wsrep_sst_xtrabackup-v2-ipv6, and altered it so that socat always uses TCP6-LISTEN. When that new plugin is deployed across all the cluster members I have SST using xtrabackup over IPv6 working.
As a better workaround I created a new SST plugin - called it xtrabackup-v2-ipv6. I merely copied /usr/bin/ wsrep_sst_ xtrabackup- v2 to /usr/bin/ wsrep_sst_ xtrabackup- v2-ipv6, and altered it so that socat always uses TCP6-LISTEN. When that new plugin is deployed across all the cluster members I have SST using xtrabackup over IPv6 working.