At the moment there is a way to extend custom actions using $g_custom_group_actions, however, it is complex since it expects the developer of the custom action to re-implement a lot of the standard handling since the developer needs to implement a new custom action form and action pages. The new infrastructure should all the user to implement a new action by simply implementing an include file with some needed functions. These functions are then called in the appropriate place from a standard framework for custom group actions.