There should be a way for me to plan sprints against a pre-defined release... I want to use my sprints to burn down my release backlog.

To do this, it would be better if there were the ability to filter by release in the bottom-right portion of the sprint planning page..

Seems like a big oversight. I understand that you can use the portfolio level to filter by feature, but if I've moved my features into a release already and want to plan sprints against that, it seems like a no-brainer.