This particular problem could be solved by allowing admins to move a message to another forum that sits at the same level in the site map. That would be easy to implement, and we'd have status quo.
The tricky part is when you want to move to another instance in another place in the site map, because you have to show the context. Say they're all called "Forum", but one sits below "Project A" and another below "Project B". You'd have to include that context for people to figure out which one they want.
This is not a super-complex problem by any means. It's just not *quite* as trivial as when we're staying within one context.