The thing there is that is already how image processing and compositing has been done for 30 years. Check out Nuke, Digital Fusion (which is free), Shake, Flame, etc.
I'm not talking COPs or Houdini is something bad. It is one of the few applications that I support with whole heart and I love it. In fact Houdini is a very rare example of really well defined visual DSL.
Luna differs in many ways, the most important are that we've got double representation and Luna is a real programming language, while Houdini provides you limited set of building, yet very powerful blocks. I would however love to see Luna used within Houdini as a plugin, I've been already speaking about it with some folks :)