'Sub editor' functionality

I have a client who would like organic groups where some subscribers can have sub editor roles whereby they can edit child group forum posts, but cannot edit the parent group node, and can't manage group subscriptions.

To get something approaching this functionality I've installed og_forum and og_user_roles, which appear to be doing their jobs fine. I've created roles which have some but not all of the permissions the group owner has. However, the sub-editor role cannot edit other child nodes, unless they have group admin permissions, or if they have the 'administer nodes' permission, isn't a desirable situation in the least.

From a look through the og module it seems that og's og_grant_access hook will return editing access if the user is_admin for the group, but not otherwise. From this it seems that og doesn't support this half-way house sub-editor functionality. I can see two ways around this:
1.) Hack the og module to provide an is_editor column to og_uid and define behaviour around this
2.) Create a custom copy of the forum module that is og aware and its hook_access implementation returns true where the user's parent group's role = subeditor.

Either way it's a pain. So... have I got this right? Are these my only options?