If there's a post that has no answers (and has a bounty offered), but has helpful comments, could the bounty be awarded to the author of the highest voted comment, or allow the OP to award the bounty to one of the authors of a comment that the OP thought was most helpful?
Perhaps the OP can just ask the commenter to make that into an answer, then award the bounty to that answer. Comments are temporary, and it doesn't seem practical to indicate that a certain comment has received a bounty award since it's not even possible. Plus, rep isn't incurred on comments at all. If that commenter isn't around to fulfill this request, someone else may end up making that into an answer to receive the award. Bounties can't be awarded to self-answers, but the OP can still provide an answer based on a helpful comment as it will help others.