Most agents probably aren't going to say "over the maximum permitted mileage" unless they think you know what they'll mean. It seems their computer doesn't give them very good information when it doesn't like what they've requested, which makes it hard for them to give a solid answer. I think every city that's on a valid MPM string of flights (where the flights actually exist) is part of a published routing between the origin and destination if there are no published routings.

Sorry for opening up that can of worms, but I am incredibly curious if it would work. I'd love for a bored PM/DM with an EF subscription to see if that routing could be made to work.