There really isn't a good way to do something like this through the firewall itself. I know that creating a custom data pattern and utilizing it within a DLP profile was a way that people have worked around this limitation in the past.
Sadly the way that Google recommends restricting this access with a special HTTP header isn't something that PAN supports at the moment, and Google doesn't offer anything to make categorizing the application anything other than gmail.
So a custom data pattern is going to be the best solution that you can really do at the moment, but that isn't the best solution and isn't without problems in it's own.
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