I know the rule says "one post, one topic" but these two points are rather tightly related.
1) a PlayerInteractsEvent to be fired when the player interacts with some object ([F] key). I could not find it even in the LUA API. Among other things, it could be useful for controlling access to objects like workbenches, furnaces, etc. in protected areas (it may subsume the current LUA event "PlayerObjectStatusChange").
2) a World3DModel::SetInteracts(boolean) method to activate the possibility to interact with a custom 3D model. Interaction should not do anything specific on the model itself, 'simply' show the hammer cursor and trigger the above event on [F] pressed. Plug-ins could process the event to implement specific functionalities.