What keywords to use when searching for a Maya plugin developer?
Your best bet is probably to look for developers of commercial Maya plugins and see if they are willing to do contract work.  Most studios have developers on staff to handle any project specific coding work needed.  It's a pretty specialized skill set and I doubt there is much of a market for doing it as consultants.  

That said, my (limited) experience playing with APIs in 3d graphics software would tend to indicate that any developer with experience working with 3d systems should be able to pick it up pretty quickly.  The concepts, when it comes to the programing, aren't that different from DirectX to OpenGL to SoftImage or Maya.  It's still shaders, geometry, physics, etc...