The implementation made by Steeffeen is an approximation of what could be considered an headshot, it's good but isn't 100% robust to all use cases. The game need to be updated to support such a feature, so it would be better to post your suggestion in the wishlist.
However as an alternative I updated the LibXmlRpc_OnHit callback with more data : victim and shooter positions and aim directions. So you'll be able to test for an headshot like Steeffeen did if you really want to.
The commit is available in the Test branch on GitHub.