Just realized.. it's not going to be good enough to add the metatag to the popup template, because that's nested in an iframe, which is agnostic of facebox. The true fix would have to be to add the emulate IE7 meta tag to the CMSMain template. Ouch.
As an experiment, please try adding this as the first line to DataObjectManager.ss (or FileDataObjectManager.ss, or whatever you're using..)
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7">
Actually, I can almost guarantee that this won't work because the DOM is loaded with AJAX. Hmm..
Alright, time to look for a new modal window. :)