I think our scheme thus far has been to declare input and output plugins here always, but then for a plugin that supports only input we have a very minimal FORMAToutput.cpp file.  For example, see src/gif.imageio/gifoutput.cpp.

I think what you've done here is an equivalent solution, but perhaps we should stick with the usual convention.

Plus, I'm tempted to think that if you make it easy to just say INPUTPLUG, the output is out of sight, out of mind, whereas if people are forced to write and submit an empty output stub, there may be benefit to the reminder that the work is in some sense incomplete.