Answer
No — &MESSAGE_TEXT's actual content is hardcoded within the framework's own Approval Routing API and can't be directly edited or reformatted. However, if the real requirement is just customizing the EMAIL SUBJECT LINE rather than the message body itself, there's nothing stopping you from building your own custom subject line directly from the other individual attributes the event already exposes, like &KEY01, &KEY02, &KEY03, and &KEY04, which are the same underlying values &MESSAGE_TEXT itself is built from, constructing whatever specific subject-line wording and field order you actually want, rather than trying to modify &MESSAGE_TEXT itself.