I think you are mistaking the Relying Party and the OpenID Provider. Using StackOverflow as an example, SO is the Relying Party and other sites (e.g. Google) are the OpenID Provider(s).
The Relying Party (e.g. SO) has no say in when the OpenID Provider (e.g. Google) chooses to "forget" the user for their own purposes.
EDIT: I was not familiar with the PAPE extension. However, if I understand correctly, it does not provide a way for the RP to say to the OP, "On your end, forget this user's authentication", which is what I think rehan wants. It just provides a way for the RP to demand another authentication for the RP's purposes.
I still see no way for (e.g.) SO to log a Google user out of Gmail after SO logs them out.
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