Basically, how can you have a custom 'edit' view for a mosaic tile?

### Long version

When using the [mosaic editor](https://github.com/plone/plone.app.mosaic) in plone 5 with a custom tile, you provide a view template to render the tile, for example:

```
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en" xmlns:tal="http://xml.zope.org/namespaces/tal" xmlns:metal="http://xml.zope.org/namespaces/metal">
    <body>
         Content here...
         <img src="${view/image}"/>
    </body>
</html>

```

This renders as a draggable tile when you edit the layout, like this:

[![example](https://i.stack.imgur.com/SutFV.jpg)](https://i.stack.imgur.com/SutFV.jpg)

However, when the content of this tile is complex, the performance of the editor becomes terrible and in some cases (eg. video and react components), completely breaks the editor.

How can you have a 'placeholder' view (eg. fixed image) that renders only when editing a page / dragging a tile?

(NB. I see inspecting the page element, that a 'WebDAV' lock is applied to the page while it is being edited, but this doesn't seem useful in any way, as it is not unique to the user's session, ie. logged out users will also seem the same page state.)