Simplest approach for this would be do the same as now, except instead of just the HTML, the full page bundle in JSON form would go in External Store.
Then, we need to extract the different parts of the content bundle. E.g. for fixed-html we just extract the HTML, then fix it.
Ideally, for VE we can just give it the different components (data-mw and HTML) separately, and let it deal with them (I think it needs to anyway due to general VE plans).