Exit trailhead or date on the permit don't really matter -- you can end up exiting where and when you want. They do want you to put that on the permit, but they really only care about entry date and trailhead.
One big challenge will be logistical, specifically food resupply. You do know that you'll need to carry bear canisters? There's no way you'll get 3 weeks of food into a canister (nor will you want to carry all that weight) so you'll need to resupply food at least once, probably twice. Mail drop for resupply could be problematic because Tuolumne Meadows post office may or may not be open during your time, and if you send food c/o the Yosemite Valley post office then you will end your permit to come off the trail to get your food (Yosemite Valley is considered front country, so as soon as you set foot there your permit ends, but TM doesn't have that condition). You may get a better handle just before your trip on when TM will open. I don't know if Wawona post office is an option for food resupply mail drop, but even if it is that limits the scope of how far you can go (you said you want to do a big loop around the park, which you can't do if you keep yo-yoing in and out of Wawona). You also could buy food at TM and/or the Valley, but same restrictions apply (and, even if open, the selection in the TM store is pretty limited). Your other option is to stash food beforehand at trailhead bear boxes, keeping in mind that you may not have access to Glacier Point Rd or Tioga Rd before you hit the trail so that could severely limit your stash options.
Keep in mind that the high country could still be blanketed in snow at that time of year, which could obscure trails and require special skills and equipment.
There's a lot of great stuff above treeline, so hammock camping could limit you a bit as well. If you are able to lay your hammocks flat on the ground and pitch the tarps without trees then that limitation goes away. But, above treeline also is more likely to be snowed in. Some trails, such as Red Peak Pass, probably will be impassable that early in the season.
As for itinerary, there's too much great stuff to see even in 3 weeks. Good thing is that you really can't go wrong no matter what you do. Did you get a Half Dome permit with your wilderness permit? You need a permit to ascend HD, and while many will argue that the views are better from other places, ascending HD is an incredible and unique experience.
Have a great time!
- Billy JoelEdited 1 time(s). Last edit at 01/17/2018 08:35AM by JRinGeorgia.