This would be nice, but goes far beyond current functionality of MW core, so marking as low priority.

(In reply to comment #1)

This would be nice, but goes far beyond current functionality of MW core, so
marking as low priority.

It's unclear to me at which stage the visual document that a user is editing is converted to wikitext.

When a user presses "Save page" (for the second time, as you point out):

• the HTML DOM gets converted to Parsoid-format HTML+RDFa,
• this HTML+RDFa is squirted at Parsoid,
• Parsoid converts it to wikitext, and
• Parsoid saves it to the DB (using the user permissions/etc. as provided by VE from the user session).

It seems like this task is sort of obsoleted by T102398: Migrate wikis to use a single edit tab which has both visual and wikitext modes and allows on-the-fly switching between them and its blockers+dependencies.