One of the challenges of implementing RP specific features is, I suppose, ensuring that you cater to the very broad number of ways to manage an RP.

In regard to co-GMs, I would suppose you could make it such that the thread creator (or a moderator) is able to give the co-GM title to other users. This would provide a little flag in the corner like you have for the OP, and it should also provide the power to edit the first posts, or at least posts which the GM decides. What I could see that being useful for is to allow co-GMs to update things like character lists without the GM's personal intervention. It may be useful for the GM to limit which posts can be edited by the co-GMs (eg. can edit Character tab, but not the IC or OOC), just for security. I suppose the wiki-style OP suggested before may (or may not) fit this role well, with the creator and moderators able to edit permissions, although it should blend with the normal post style (I suppose you could just borrow wiki code, then. But then compatibility with old threads?).

Speaking of security, if such a system of shared ownership is implemented, a short-term post backup would be useful, to help restore griefed posts.