After the discussion over in custom modals/dialog and with Niels suggestion to perhaps use some other form of UI/UX to solve the problem than a dialog/modal.
Having the dimmed overlay ‘shielding’ the content a little more visibly than the way Workflow slightly greys out the field is a good touch. But there is a nicety in leaving the content tabs available to interact with. It would allow users to navigate through all the other content sections if they were trying to reference a locked one for authoring of a different page for example.
Aw dang! I’ve only played around with up to v13 so didn’t know it was just gone in v15. That’s unfortunate. It was such a nice’n noticeable thing for authors who aren’t the most tech savvy and aren’t really attentive to tiny notes or icon-only indicators. With just the save/publish options replaced I can already hear a buncha clients asking why they can’t edit xD
In v13 it just appears on the Content App view when looking at a page and you don’t need to explore anywhere else to see that little alert. So it’s nice and upfront without being too disruptive.
Short of being able to plop something into the document workflow… The top would be the nicest and first-seen locations for a content lock indicator. However, being that it can be so different depending on the available options and setup, perhaps placing an indicator around where the save/publish button would be is the next best? It’s a more stable area in terms of the space around it without risk of covering something of interest, and is a place authors are used to looking at for actions.