Re: plugin idea: wiki post (group editable post)
If you have to add it to the topic, messing with the post order could get ugly. Probably easier to just leave it as an option when creating the topic. You can then add an entry to topicmeta, and use a hook to check if the topic being called is such a wikitopic and add to the roles available to the current user (giving him the edit_topic or rather edit_post capability). That should let him and therefore anyone edit it.