Right now you can say alwaysFollow="TRUE" which means do not involve the user if there is any cookie. The first one (== most recent) is selected.
The idea here is that if you say alwaysFollow="SingleEntry" it will select the entry iff there is only one. The useful thing is that I never published an XSD for the cookie plugins configuration so we have a seamless compatibiloty story.
There is a usecase for this in EDINA (to do with multiple services and trying for a seamless SSO).
I'm slightly concerned that this may start interacting with the isPassive protocol control, but so long as iPassive takes precendance we should be OK.