I am working in science near all my life, so everything I normally write is open source by definition.
But the plug-in is an addition to commercial, not open source and relatively expensive program.
Original idea still was making the source open. But once I have understood that I can not compile CW code out of the box without spending ~$300 on compiler, I have changed my mind.
Note, that I do not use the CW code at all, only COM IDL. They have released the code under MIT license and you can find direct replay to me from CW that I am free to choose license I want.
Looking at related projects (BCF, etc.) with open source I do not see that helps in any case.