Feature summary:
Modify Extension:CodeMirror to work in Extension:ProofreadPage's headers and footers.

It doesn't look very complicated, e.g. stuffing the code given in the doc to add CM to a textarea, looped for the three textareas, in a browser console, works.

Use case(s):
All pages in Page: namespace on the Wikisources.

Benefits:
The same benefits that CodeMirror has in the first place.