On Thursday there was the announcement by Kristian -- and the initial code -- for what is becoming a Weston SDK. "It's an out-of-tree weston plugin. It's just a silly little overlay that you can pop up with mod-space, but the interesting part here is that it's building outside weston. Current, that works by copying the header files that defines the weston <-> plugins API, but I'd like to start thinking about how to formalize this process. I don't think it should be a big problem."
For now though this Weston SDK out-of-tree work won't have a stable API across major releases since he doesn't want to slow the internal development process of this reference compositor to Wayland. He's also unsure right now how much of the Weston API to expose externally or how useful it will be, but this is a first-stage concept.
The "standalone plug-in" for Weston is currently living in this Git repository.