The onActionSimpleRepoAction function in share/components/documentlibrary/actions.js defines the wrong webscript URL, if the calling action wishes to execute asynchronously.
The ActionQueuePost webscript accepts a webscript parameters, async, which ensures that the action being fired is executed asynchronously. The only way to add this webscript parameter from onActionSimpleRepoAction is to add an additonal URI template variable to the webscript name.
Line 1042 in actions.js should have:
With this change, an action defined in share-config-custom.xml can include an async parameter, and have that parameter passed through to the Action Queue webscript, enabling asynchronous execution.