I do not think Spark should support non-lts releases. Especially for JDK9 I do not see a strong technical need, but maybe I am overlooking something. Of course http2 etc would be nice for the web interfaces, but currently not very urgent.
May I ask what is the use case? Although it is a very interesting question, but I would be concerned about going further than a proof of concept. A lot of the enterprises I see and visit are barely on Java8, so starting to talk JDK 9 might be a slight overkill but if you have a good story, I’m all for it!