Sorry, I don’t have any clue. I converged to the solution I proposed by trial and error. I am still new to the world of Binder and packaging.
I also tried at first to use the conda-forge octave package. And I tried to check if there was any way to make the C++ compiler work when I try to install Octave package on the go. That would have been a more flexible solution indeed. However, I also didn’t know where to start looking.