Is it possible to create a Gmail filter that works on arbitrary headers?

(Not a dup: this very similar question really asks about the DeliveredTo header).

Bugzilla provides a number of headers telling you why you received a mail, such as X-Bugzilla-Reason. How can I filter on arbitrary tags such as this one in Gmail?

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I'm offering a bounty; I really need this. (Bugzilla is my use case, too.) –  Dan Fabulich Apr 26 '11 at 0:57
I want this for paypal's email-xclick-payment-notification header, delivered in the header X-MaxCode-Template. Sigh. –  Bryce Jul 29 '12 at 14:47
Not being able to filter by a custom header has been pain for me for a long time. That's why I've decided to develop FilterMyGmail, an application that will address this and other related issues. –  jcollado Jan 26 '14 at 0:48

However, you could use an intermediary running procmail1, preprocess the mail and add something to the subject, or change its sender, so that it can be filtered secondarily in Gmail.