APM clientless mode

"this still cannot be configured with a "configuration variable" in the VPE? Any clue when that will be possible (RFE...)?"

I don't believe that's ever been asked for. You're welcome to open a support case and request it though. The clientless-mode header is a "flag" that is sent to the APM filter processor before it starts a new session (initiated by the 302 redirect), and there are absolutely reasons why you'd want to be able to enable and disable this dynamically in a single VIP. For example, versions of the Exchange iApp/iRules use clientless-mode for different agent and application types. And for what it's worth, the iRule code is pretty simple:

when HTTP_REQUEST {
    HTTP::header insert "clientless-mode" 1
}