Hi there Graeme,
Right now we don't have any news to share regarding an SDK for Storyline, but yes, it is on the radar and is something we do plan on offering at some point. The Word Of Mouth blog is the official channel for sharing news of that sort, so feel free to subscribe to it for the latest info.
Regarding player tabs: as you mentioned, Storyline allows you to turn on or off the following tabs on a slide-by-slide basis: Resources, Menu, Glossary, & Notes ... but currently, if you add any custom tabs to your player, those will appear for all slides (it's not currently possible to turn them off on a slide-by-slide basis). That would make a great feature request, though!