Machine code can be decompiled too, even if not as cleanly. It would slow down efforts to make an unauthorized fork of the software as a whole, but anyone curious enough about individual bits and pieces could still see exactly what's happening. Closed source is really more of a legal block than a technical one.

Given that and the lack of popular, high-level languages that compile to machine code (C++ is the best I can come up with), I'd reconsider whether that's really a priority.