Developer Steve Troughton-Smith has discovered that Apple Watch complications seem to run remotely as extensions (out-of-process). This leaves open the possibility for Apple to give developers access to the necessary APIs to build complications into their apps.
I continue to believe (as I wrote last week) that Apple will slowly give access to more and more customization features and APIs over the course of the next few years, similarly to how they developed the iPhone ecosystem. We aren’t going to get everything all at once and the Apple Watch that you have on your wrist in 5 years will be drastically different in both hardware and software than the one you have today.
(Via MacStories Weekly.)