I also considered the creation of a new request header as well, allowing the browser to signify that it is micropayment-aware (without promising payment for the request). This would allow the server to keep stats on how many clients are signalling support for micropayments, and possibly to serve different content depending on its presence or absence.
However, there would be nothing to stop a client from sending this header regardless of whether or not it actually facilitates micropayments, so in the end I'm not sure how useful it would be.