The key in the not so ubiquitous Exit Permit...and believe it or not there are work around that flirt with being legal, if you can't get one on walk-in.
Personally, I just request an available permit on a day closest to my exit date. This avoids the hassle of explaining why a trip you planned to do in 3 days morphs into a 6 day trip in the matter of minutes.
My belief is the forest doesn't give a rat's rear end when I exit any other trail, why should the give a rat's rear end when I leave the MMWT...yeah, I know being the one extra person on the MMWT I am ruining ~200 people's wilderness experience.
With that said, my guess is if you really want to use the MMWT the whole trip, there will be a permit for when you walk in to the ESIVC, come early and be prepared to walk on the trail the same day.
