When generating a ParserOutput object for the content of one slot (via ContentHandler::getParserOutput), it should be possible to access the Content of other slots in the same revision, to allow cross-slot data access (e.g. transcluding information from some auxiliary slot into the output of the main slot). This should work for atomic edits too: when previewing an edit that changes slots X and Y, and the content of slot X uses parts of Y, the new content of Y should be visible in the rendering of X (as well as in the combined rendering of all slots).
An initial idea is to pass RevisionSlots (including inherited slots, after PST) to ContentHandler::getParserOutput. The relevant code is in WikiPage::prepareContentForEdit (resp. PageUpdater::prepareContentForEdit).
Note: This does not block SDC General, since SDC doesn't use atomic edits, and Wikibase doesn't rely on MediaWiki's transclusion mechanism. For cross-slot access as needed by SDC, see T194730: Ensure that Wikibase data access to MediaInfo on file description pages works with MCR .