I have a bunch of separate interior cells, if I lump them all together into a single interior, and connect the navmeshes through the doorways (see pic below), and those doorways in the exterior are fairly far apart, theoretically you could make that work, right? If the follower NPC follows you out through whatever door corrrectly, followers would be alright, and if there are no "wandering NPC's in said interiors, I don't see an issue. Any thoughts? I haven't tried it yet, but I'm thinking I just might. This would definitely save load time if those rooms are repetedly entered/exited.

http://i.imgur.com/UKb20.png