I recently posted a discussion based question and got tons of very good answers, so I'm wondering if it's possible for me to specify a reason for accepting a specific one? If there's nothing built in for that, would it be considered acceptable (rules and etiquette wise) for me to edit my original post and add the reasons I picked a certain answer at the bottom of it?
Generally, the usefulness of answers is rated by upvotes as well as whether or not they're accepted. If the accepted answer happens to be highly-upvoted, an explanation might not be necessary because it's clear many people find it useful.
If you choose to accept an "obscure" answer with relatively few votes, a comment on the accepted post doesn't hurt. You shouldn't feel obligated to explain what works best for you, but it might also help others understand how you arrived at your conclusion. If I'm looking at a low-voted accepted answer, I'm interested in why the author's evaluation seemingly contradicts most voters. That explanation could help me if I have the same problem.
It wouldn't be a great idea to edit your conclusion into the question imo. People still post new answers to old questions even after an answer has already been accepted - so adding that degree of finality to the original post might prevent new insights in the future. Also, your explanation of choice is arguably more relevant to the answer itself than to the question.
It's still OK to put your reasoning in the body of the question. People edit questions to explain bounties, providing some degree of question-answer discussion on the original post. It isn't much of a stretch to extend that process to an accepted answer.
TL;DR I don't think it's against the rules to provide a reason in the body of the question, but a comment makes more sense.