Currently, if the --defaults-group-suffix option is not set, then MariaDB checks if the $MYSQL_GROUP_SUFFIX environment variable is set. If it is set, then MariaDB uses that value as the option group suffix:
https://github.com/MariaDB/server/blob/mariadb-10.4.11/mysys/my_default.c#L237
https://mariadb.com/kb/en/library/configuring-mariadb-with-option-files/#custom-option-group-suffixes
To further MariaDB's branding, I think MariaDB should similarly be able to use $MARIADB_GROUP_SUFFIX to determine the option group suffix, if the environment variable is set.