That solution does work, but it points out a really frustrating aspect of designer and it's relationship with studio -- that the current decision to make Studio only accessible by those with DFP Premium completely neuters the value of using Web Designer and drives small companies back to using Flash.
For example, I just created an HTML5 for a local indy newspaper that featured an embedded video. The video is set up to autoplay with the sound muted unless the cursor is over the ad space. Everything worked great until we implemented it as a third party creative in the iFrame. The problem is that adding the macros created two CSS boundaries "on top" of and still "outside" of the ad space. So while watching the video with sound, if someone roll's over either of the two links, the audio is muted. More on this problem here:
https://productforums.google.com/forum/?utm_medium=email&utm_source=footer#!msg/dfp/IXTHd-xNXr0/b-qoxE4lr-0J
This is crazy! Especially since the add is built with all of the necessary tags for DFP already embedded within it.
I appreciate the desire to keep Studio a premium feature, but the fact is most web publishers don't meet the necessary traffic or click through levels to qualify to use it. And so we have to create these workaround that end up not allowing us to take full advantage of GWD or just giving up and going back to flash (which DFP will try to convert back to HTML5 anyway).