Can anyone tell me please?

Do plug-ins made by Mac OS Automator workflow, using “Save As Plug-in… menu” create a plug-in script that will be available — and work — directly in the DA interface?

Yes and no. You can add workflows to DEVONagent’s script menu (by saving the workflow inside the folder ~/Library/Application Support/DEVONagent/Scripts) but you can’t create contextual menu plug-ins that way (those plug-ins are actually Finder plug-ins).

Thank you Christian.