Checking the response headers from profiles.wordpress.org
Ick. We can do better than that.
Rather than hitting the profiles directly, have you looked at how we display gravatars here on places like Trac, which don't have access to the w.org user tables directly?
Compare the results of these two requests:
https://wordpress.org/grav-redirect.php?user=BrashRebel
https://wordpress.org/grav-redirect.php?user=non-existent-username
We may create an API for better user information in the future, but if you're going to do this as a hack for now anyway, I'd rather you hit an endpoint that a) isn't likely to change and b) is very low cost for us. Hitting profiles directly, not very low cost.
Just make sure you label the hack in the source so that it can be understood later.