I stumbled upon a performance issue and fixed it in my fork. It's fairly simple code, but an additional API in liballoc, with choices to be made. I gather from the RFC guidelines that this should be done through an RFC. BTreeMap::get_key_value seems like a similar case, and it was stabilized quite recently (1.40.0). Its corresponding commits and pull request don't seem to mention an RFC but a tracking issue.
So should proceed with a PR, an RFC, something else, or not at all?