Suppose you forwarded the message to yourself; that would allow you to edit the title, although with the limited space on the title line, not much information could be posted. You could use code numbers to represent some data, to lessen space needed. You could add any extra information needed to the text of the forward message while keeping the original text (and heading) intact below.
Pegasus Mail, a mail client, has an annotation feature whereby one right-clicks a message and can add or edit a note. The note doesn't show until the message is right-clicked again or the message opened and the annotation tab clicked, so isn't immediately apparent as you seem to want. Messages with annotations are marked in the folders though.