Interestingly IETF discourages the use of X prefixed headers but they might still suite this kind of behavior http://tools.ietf.org/html/rfc6648
*I'm not trying to be pedantic, genuinely curious if it is a word I do not know.
Edit: on second thoughts, it's the IETF, they always move quick. And it's not as though this was a particularly thorny protocol or anything.