Force MariaDB Galera to only create InnoDB tables
This is a good question. People tend to want that when using NDB cluster and Galera.

The right answer is that you should report the plugin creators to create the tables with no engine definition, so you can set it with "default_engine" option. While on other cases you could disable the engine and force the engine substitution with the default one, that is not a good practice, it is discouraged on the latest versions and impossible with MyISAM.

The truth is that once the developer writes ENGINE=X, it is forcing the creation of the engine, as written. The only way to accomplish that is by rewriting the query on-the-fly. That is available in MySQL 5.7 (not yet GA), with several plugins. The only way I can think of doing it now is with a SQL proxy like ProxySQL or MySQL Proxy. Please note that some of those are infamous for bad performance, so you may want to enable it only on plugin activation/disabling.

There is obviously another option: download the MariaDB Cluster sources, disable MyISAM on the CREATE TABLE Syntax, and compile!