How to create a select list of which the selected item is used as parameter in VBO rules component? It's a common scenario, but I couldn't find a solution yet:

I have a view (a list of tasks) with checkboxes (views bulk operations)

I want:
1) a dropdown select list of taxonomy terms 'Priority' above the view.
2) a rules component that uses the selected taxonomy term, as parameter for applying the priority to the selected tasks.
VBO uses the core 'actions'. You can create new actions with hook_action_info.  

The https://www.drupal.org/project/examples module has a good action example.