In ArcView 3 all controls (menu choices, buttons, tools) had scripts associated with them that could be changed simply by supplying a script with the same name through an extension (plugin). That way you spared designing a new control and could keep your panels tidy while improving and extending functionality of the program.
Now I'd like to do the same thing with QGIS: E.g. I'd prefer to preselect the default directory of the "add ogr layer" action by the path of the first active layer in my map.
So if it's possible I'd like to replace or modify the mActionAddOgrLayer.triggered Script, as long as my plugin is loaded.
Another way to achieve this would be to modify the built-in system response to a change of the active layer(s) by changing the variable that by default saves the directory of the last file added. I'm sure there is a signaling but I don't know if and how responses can be changed or added without adding additional gui-controls (actions).