Currently, LivePreview only passes the $wikiPreview to the hook
https://github.com/wikimedia/mediawiki/blob/6709a1fd082bcbc1b89d499edeafa04fd2e8a517/resources/src/mediawiki.action/mediawiki.action.edit.preview.js#L101
but the script changes the page content in many other places:
https://github.com/wikimedia/mediawiki/blob/6709a1fd082bcbc1b89d499edeafa04fd2e8a517/resources/src/mediawiki.action/mediawiki.action.edit.preview.js#L31-L43
The new content for these parts should also be made available to the user, by passing them to the hook (as VisualEditor does, for example
https://github.com/wikimedia/mediawiki-extensions-VisualEditor/blob/ccdc215a9882d6bb3e079ca822f64c9089c0d363/modules/ve-mw/init/targets/ve.init.mw.ViewPageTarget.js#L1574
).

This would allow scripts like
https://en.wikipedia.org/w/index.php?title=MediaWiki:Gadget-featured-articles-links.js&oldid=619815637
to update the modifications of the language links after the user makes some changes and use the live preview (in the example, the "badges" for featured/good articles in other languages).

Also, this is likely what UniversalLanguageSelector needs in order to fix bug T53029.