Request Logging Other (APM) Information

The request logging profile is a little odd with an APM profile, as it generally triggers on every request, even the ones that should be hidden inside the APM access session evaluation. That said, you could technically create a set of HTTP headers that the request logging profile can consume, and then delete them just before sending the traffic off to the server.

when ACCESS_ACL_ALLOWED {
    HTTP::header insert "USER" [ACCESS::session data get session.logon.last.username]
}
when HTTP_REQUEST_RELEASE {
    HTTP::header remove "USER"
}

And then in the request logging profile, you'd just capture ${USER}. I think though, for the complexity of this, that you're probably better off doing HSL in an iRule to capture what you want.