Make header fields configurable

It would be very helpful if message_notify provided access to header fields in addition to From and To.

I am trying to use it with OG, Mailhandler and Mail Comment to allow users to send emails that are turned into nodes, from which messages are created and sent to group members. Group members would then be able to reply by email, creating comments on the node and generating new messages to the group.

The problem is linking the comment emails to the original nodes. I would like to set a header field in the outgoing message from which feeds can look up the NID of the node, and possibly the PID of a comment to which an email responds.