While not a total solution, it seems like it might be worthwhile forcing flake8 or similar checks when uploading PyPI modules.
That would catch the illegal escape sequences where it really matters - before they enter the ecosystem.
(general) fathead:pyxll-www sholden$ cat t.py
"Docstring with illegal \escape sequence"
(general) fathead:pyxll-www sholden$ flake8 t.py
t.py:1:25: W605 invalid escape sequence '\e'
while this won't mitigate the case for existing packages, it should reduce the number of packages containing potentially erroneous string constants, preparing the ground for the eventual introduction of the syntax error.