A new HTTP-Based OpenID Connect Logout spec has been published at http://openid.net/specs/openid-connect-logout-1_0.html. This can coexist with or be used instead of the current HTML postMessage-based Session Management Spec.
The abstract for the new spec states:
This specification defines an HTTP-based logout mechanism that does not need an OpenID Provider iframe on Relying Party pages. Other protocols have used HTTP GETs to RP URLs that clear cookies and then return a hidden image or iframe content to achieve this. This specification does the same thing. It also reuses the RP-initiated logout functionality specified in Section 5 of OpenID Connect Session Management 1.0 (RP-Initiated Logout).
Special thanks to Brian Campbell, Torsten Lodderstedt, and John Bradley for their insights that led to some of the decisions in the spec.