In case anyone else finds this post while looking for an answer, I did find a workaround:
When you are NOT in WebView, right click on the sheet, and go to Properties > Objects. Find your extension object in the list (it may be listed as a bar chart or something random since QV doesn't actually know what it is), then go the properties of the object from there. You should then be able to go to the layout tab and change the layer.
If anyone has the actual answer of how to get it into the Properties page of the extension object, that would be even better.