The problem this DOESN'T solve is that if the resource is not availale over SSL, then this won't work at all and will show broken links.

I've got a project that addresses this by converting any URL to SSL:

  http://www.fixweb.co/  

Just take whatever URL you want to access, like

  http://example.com/test.gif

add the FixWeb.co address in front of it like this:

  https://fixweb.co/example.com/test.gif
  ^^^^^^^^^^^^^^^^^^

and it will return the file over SSL.

It's not designed for high security file delivery, obviously, but it will get you around a normal Mixed-Content warning.