Thanks Radu, seems promising,
I have been able to install the performAuthorActionOnProjectResources plugin and to edit the config.xml file in order to point to my own Dita Author action ID.
I built a project where I added the batch of Dita xml topics I need to be processed. In the project window I select all the topics, then right click, and run the new menu item made available in the Project contextual Menu thanks to the above plugin

.
I got a " 'mymenuItemName' finished " message in Oxygen so I know the process has been completed and I can see that the "Date modified" properties of my topics are updated in windows explorer, however when I open the topics, I don't see the modifications I would have expected.
I tried again with a more simple Dita Autor action (without js operation, just using standard InsertFragmentOperation) but same observation that the topics, despite being edited and saved, do not seem to have been processed with the invoked Author Action.
I also tried to put a pop-up in my Author Action, the pop-up appears when I run the Action from the Author toolbar but I have no pop-up if I run the Author Action through the new Menu Item from the Project contextual menu.
I put a trace within the plugin wsAccess.js file in order to retrieve the replaceReferenceContentAction variable and it returns what seems to be a valid Action ID = ro.sync.ecss.extensions.fb@6d11c (I say valid ID because I also tried altering on purpose the Action ID in the plugin config.xml file and it this case I got an empty replaceReferenceContentAction variable)