Hi Markus Berchtold,
Thank you for your suggestion.
Fortunately there is a way that you can add this to the RadEditor. You can subscribe for OnClientPasteHtml event. Then check whether the CommandName is "InsertFlash"
and get the iframe content using args.get_value() . When you have the content you can add the desired attribute, but you need to parse it. Finally you can use args.set_value(newContent) to set the new content. More information at OnClientPasteHtml
the Telerik team