From a quick investigation it looks like there are about a dozen different ways in which VisualEditor can reasonably find out that the page is not a real wikipage.

Not one of them is being used by EP right now.

* It's pretending to be a wiki page (by having its own namespace and using action=view, albeit overridden, to render the dynamically constructed page).

* Generally this is something we have Special pages for. If the rendering is completely taken over (as in, there is no page table entry, no revisions table entries etc.), it should be a Special page. Or at least a custom namespace with a negative namespace id.

* It abuses existing WikiPage action queries (action=edit, action=delete), which doesn't make sense because it isn't a WikiPage. So inherently this is going to cause trouble because:
  1) They aren't compatible (the query string parameters Action pages take aren't supported, other than title=).
  2) It doesn't scale. Right now they take over move, delete, edit and view. But there are more page actions, and by design they will not support all of them (they override the ones they re-implement and the rest just fails). For example "View history" (action=history) is quite useless right now. And action=edit doesn't work as expected. Not to mention API actions, none of those are working as expected.
  3) Existence check impossible.  Because they aren't actually wiki pages with page and revision ids, existence check isn't possible. ContentModel can't be overridden because it doesn't use wikipage content. All pages are considered inexistent pages in a custom namespace, and then overridden to exist in some cases.

There are many different ways in which EP could indicate in a standard / reliable way that doesn't require other code to hardcode for EP specifically, and it doesn't seem to be using any of them.

I'd say EP should either let us know how to generically detect it without being EP specific, or it should implement support to at least do one thing right. Possibly we could do the latter ourselves (maybe Alex is interested in patching EP).