I just dug into this because it's a neat idea but I think we're hitting the edge of what folks have considered on the web. ;) It looks like this behavior is defined by the spec to share the origin policy of the parent but most browsers haven't implemented it that way. https://code.google.com/p/chromium/issues/detail?id=58999

Using this gist (slightly modified from that bug report): https://gist.github.com/umbrae/f2251bc6c33b46210d97

* Firefox works and shares origin policy
* Chrome fails and throws an error about differing origin policies
* Safari fails with a JS error about contentDocument being null

So I think this would be pretty hard to rely upon just yet.