This must be a tricky one - even for the guys at Episerver :)
Check out Marijas guide for extending the links and modify that a little
There are just a very few good reasons to have links in open a new window so we usually train the editors to append the content, typically "link text (opens in a new window)", themselves in those rare cases.
For styling those links a CSS selector for target="_blank" is usually enough.
If you still need to modify the markup the save events or template rendering should work in this case since you don't need individual markup per external link.
Then maybe I would consider a TinyMCE solution.
I am currently using Episerver 9 (the latest version) and I would like to update the TinyMCE Text Block control (WYSIWYG).
I would like to add a class and extra Markup to an External link which will tell Blind users that the link will open a new window and take them away from the site. in the Insert/Edit link option I can add a link Title but I would like to add extra markup.
Is there a way for me to make this possible?