I've got other hosting on an unrelated server and keep off-site backups of the whole forum, so having an emergency mirror is easy to deploy. Fortunately I never used it and therefore it was an exercise in wasting time, since just keeping the database backup was enough to deploy a quick fix anywhere else if I needed it.
The only issue though is if you do need it, you get new content posted elsewhere, then revert back to your original (a bit messy) so your idea of just a standby but otherwise blank forum makes more sense. Sorry when you used the term backup I assumed you meant a sort of clone of your current forum but on a third party host and could not understand the point of it.