there aren't any walk up permits from the valley anymore for JMT exit over Donahue. It is reservation only.
. There are walk up permits from TM for Lyell canyon.
Now I will echo something, the JMT is not the best of the Sierra. It stays low and only crosses passes. An enterprising person can put together a better hike. But it is pretty damn nice.
the absolute best thing about the JMT is it's a large wilderness with no roads and few people. At least it used to be a few people. When everybody is moving north to south you don't see those other people and even though there's 50 people a day you feel like you're alone.
Everybody hiking nobo because that's the only way they can hike it right now ruins this experience for others. they also ruin it for themselves cuz they're going to cross paths with many many people per day.
Personally I wish they would not allow northbound hikes on the JMT period. It ruins what is really a special and unique chance to experience the Sierra wilderness the way it should be experienced. Solo. Alone.
End of soap box.