Use the string functionality of sh to split the address:

IPv6 test:
. scripts/wsrep_sst_common.sh --address [ipv6]:port/module/lsn_version/sst_ver/some_future_option1/some_future_option2
$ set | grep WSREP_SST
WSREP_SST_OPT_ADDR='[ipv6]:port/module/lsn_version/sst_ver/some_future_option1/some_future_option2'
WSREP_SST_OPT_AUTH=
WSREP_SST_OPT_BINLOG=
WSREP_SST_OPT_BYPASS=0
WSREP_SST_OPT_CONF='  '
WSREP_SST_OPT_DATA=
WSREP_SST_OPT_DEFAULT=
WSREP_SST_OPT_EXTRA_DEFAULT=
WSREP_SST_OPT_HOST='[ipv6]'
WSREP_SST_OPT_HOST_UNESCAPED=ipv6
WSREP_SST_OPT_LSN=lsn_version
WSREP_SST_OPT_MODULE=module
WSREP_SST_OPT_PATH=module/lsn_version/sst_ver/some_future_option1/some_future_option2
WSREP_SST_OPT_PORT=port
WSREP_SST_OPT_PSWD=
WSREP_SST_OPT_SST_VER=sst_ver
WSREP_SST_OPT_SUFFIX_DEFAULT=
WSREP_SST_OPT_USER=

IPv4 test:
. scripts/wsrep_sst_common.sh --address ipv4:port/module/lsn_version/sst_ver/some_future_option1/some_future_option2
$ set | grep WSREP_SST
WSREP_SST_OPT_ADDR=ipv4:port/module/lsn_version/sst_ver/some_future_option1/some_future_option2
WSREP_SST_OPT_AUTH=
WSREP_SST_OPT_BINLOG=
WSREP_SST_OPT_BYPASS=0
WSREP_SST_OPT_CONF='  '
WSREP_SST_OPT_DATA=
WSREP_SST_OPT_DEFAULT=
WSREP_SST_OPT_EXTRA_DEFAULT=
WSREP_SST_OPT_HOST=ipv4
WSREP_SST_OPT_LSN=lsn_version
WSREP_SST_OPT_MODULE=module
WSREP_SST_OPT_PATH=module/lsn_version/sst_ver/some_future_option1/some_future_option2
WSREP_SST_OPT_PORT=port
WSREP_SST_OPT_PSWD=
WSREP_SST_OPT_SST_VER=sst_ver
WSREP_SST_OPT_SUFFIX_DEFAULT=
WSREP_SST_OPT_USER=