I pushed this commit, which will keep the /bin and /lib subfolders only on MacOS:
Users facing this bug should just have to update Brainstorm and re-run the computation of a OpenMEEG forward model, it should update everything automatically on the OpenMEEG side.
I haven't tested this fix, I'm working blindly because I don't have access to a mac at the moment. I'd need someone with a MacOS system to try it.