Of course I will implement one of my own proposals. This would solve the problem of “no hook for an editable install” without addressing the “editable installs aren’t 100% accurate” problem and without requiring a distutils refactor. Setuptools would be able to stop putting egg-links in site-packages because pip would take care of doing an equivalent job.
Flit’s implementation suggests that both kinds of editable installs can be useful. I would be happy to see a second or an extended hook for the “only the files that would be installed” feature as an option. I don’t think enscons or distutils will easily yield that feature. I understand if you feel that implementing the simpler option would kill the enhanced option.