I was wondering if/how it would be possible to limite certain actions to a given space or only usable by certain users.
Example: I would want a "Publish" Action that does essentaily the same thing as an aprove action, but also runs additional logic. I only want this option to apear on a certain space (the one right before it goes to the finished folder) or available only to the created Publisher role.
You can certainly implement a custom action evaluator in which you can test for any scenario you like, for example only return true if the node the action is for belongs to a certain space.
There are plenty of examples of action evaluators, have a look in the org.alfresco.web.action.evaluator package.