While not yet bookable via IHG, I noticed that some Six Senses properties (the ones I saw were the two in Singapore) are now showing up in search results, so it looks like they’re making some progress on integration; hopefully IHG Rewards participation is coming soon.
The two Singapore properties (Maxwell and Duxton) are now bookable via IHG, though so far there doesn’t seem to be any indication on when they will be participating in IHG Rewards.