In my experience (development of a plug-in using the GEF framework), Eclipse plug-ins are written in Java. However, nothing prevents you to write it in another language that allows C bindings, and make it available as a shared library (.so or .dll); calling functions is then done using JNI (Java - C bindings). Which is possible with OCaml since it allows callbacks from C.

As for developing an IDE from scratch rather than use Eclipse, being given the number of features provided by Eclipse, IMHO it is not only like reinventing the wheel, but more like reinventing the car itself...