Started working on this. Technically we cannot call it "Storm Sink", as Sink's lifecycle is managed by Flume, whereas Storm Spout has it's own lifecycle managed by Storm.
It would need to be build as some kind of connector between Flume Sink and Storm spout. If we have Kafka Sink, Kafka spout can be used. Or we use an embedded agent inside Spout, and drain the Channel inside nextTuple() method.
Shall play around a bit more with Storm and try to explore other approaches as well. If anyone has got some ideas, would be great to discuss.