If you write code that you give away, your project is technically open source. After all, you don't have the code that your CPU uses to translate x86 instructions into its internal opcodes, but you can still use an open-source operating system on top of it.
In this particular, an interested party would only have to rewrite one library, instead of one library and one application on top of it. So it's a win, even if it's not GNU-certified Free Software.
I'm not claiming FaceOSC isn't open source just because it depends on proprietary software. I'm just trying to clarify the situation — open-source software that I can't practically run is a lot less exciting than what I originally thought this was.
In this particular, an interested party would only have to rewrite one library, instead of one library and one application on top of it. So it's a win, even if it's not GNU-certified Free Software.