Under the assumption that I'm far enough out that I need to hike out on my own, and that I have someone who can help... what is the best way to immobilize a dislocated shoulder for a long hike out of the woods (5+ miles)?
You don't want to immobilize a dislocated shoulder. You really need to fix it. Even if held at just the right angle and not being jostled around, a dislocated shoulder hurts. After a while the muscles will tighten, then start spasming periodically. Then it really hurts and it becomes much more difficult to get it back in.
There are some dangers in getting a shoulder popped back in by a non-expert, but the downsides and practical near impossibility of hiking out any distance with a dislocated shoulder outweigh them.
Once the shoulder is popped back in, the person will immediately feel better. It will be sore and will be in danger of popping out again easily, but it won't hurt anymore. Now is the time to arrange some sort of sling to keep the lower arm roughly accross the belly and tied down so that it isn't used, even if accidentally. Little things we normaly can take for granted can pop it out again, but not when the lower arm is held accross the belly.