Currently, the third party stakeholder would need to be a member of the workspace in order to receive notifications of replies to their comment. They can be added to the workspace as a “viewer only” meaning that they’d be unable to make edits to the projects contained within the workspace but they would then be able to set their own notifications preferences for those projects.
In any case, I’ll go ahead and show this post to our product team to let them know that you’d be interested in that kind of functionality.