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http://learnwebdesignonline.com/allow-script-iframe-tinymce...
What I don't know is where you'd put this to make it work for Concrete5 -- maybe in Dashboard -> Sitewide Settings, click the "Custom" option under "Rich Text Editor"? Or maybe there's a file in concrete/elements/ or in concrete/blocks/content/ ?
Also I know they updated TinyMCE with the new Concrete5.4.2 -- if you're still running on 5.4.1.1, it might be fixed with that new version (but I don't know for sure).
Wish I had a more complete answer -- hopefully someone else can chime in with the second half of instructions :)