You can now use the Artefact Plugin Template by Technische Universität Darmstadt, Germany, to program your own plugins for Mahara.
It provides the basic structure and contents of a plugin, defined in the Mahara Developer Wiki (Link).
Simply replace "plugintemplate" with the name of your plugin, (file names and file content) and add the author and institution.
To install the plugin put the folder into the "artefact" folder of your Mahara system. Then go to "Administration" -> "Extensions" -> "Plugin Administration" and click on install (next to your plugin).