I reread the patch and my previous comment and am no more positive than I was then.

The one use case presented is second-hand.  Chris said "in order for [Bazaar] to use a custom sequence matcher, they had to essentially copy-paste and modify the stdlib diff methods in order to inject their own sequence matchers."  Assuming that they did something like the patch, it is not clear why they did not instead patch/replace SequenceMatcher, and they are not involved here to speak.

Enhancements need to be of some general use with multiple use cases or users.  When I added the SequenceMatcher autojunk parameter in 3.2 to disable the autojunk heuristic, there had been multiple reports of how it caused problems.  There has been no additional support given here, including none from Tim.  One place to get other on board for enhancements is python-idea list.