Hi,
are there any resources on how to make the content/value of custom tags created via tag extension (Manual:Tag extensions) edit-able inside VisualEditor?
Topic on VisualEditor/Feedback
No documentation exists on that. Can you share a URL to a (public) page that uses this feature?
I sadly do not know any wiki which has this implemented.
If the tag is correctly registered as a parser tag with MediaWiki, Parsoid will (currently) hand off rendering to the PHP parser, and VisualEditor will let the user make changes to the tag's contents with a general "MediaWiki extension editor". You can sub-class this with little modification (see e.g. the hieroglyphics extension) or go all-out and make a different kind of editor (see e.g. the "Math[sic]" formula editor).