Have you seen this forum? http://www.outsystems.com/networkforums/viewtopic.aspx?topicid=3666
Can you tell us why you need this, please? What are you trying to accomplish? This way we can understand your requirement better and provide some solution.
This is only possible for public actions. The system tables Espace_Version_Signature and Extension_Version_Signature have a xml representation of all the elements the eSpace and Extension exposes as public, including the public Actions.
Note that this xml representation is internal and not supported as a feature. This means it is subject to change between major versions of the Agile Platform without notice. Meanwhile, this should tackle your requirement. Simply create an Extension to parse that xml and get the list of Actions from any eSpace or Extension.
We'll include in our backlog a change request to also store in the metamodel of the Agile Platform the Actions (and other elements) of each eSpace, so that you can get a clean API for such requirement in a future version of the Agile Platform.