Re: Our cumbersome mailing list workflow

 Editing text files isn't that hard, we do it all the time.

It is not indeed. But doing it all over again and again is hard and error prone.
I did re-read the man page on git format-patch and found the --notes
option, which I am going to try
to use in my workflow. That way I only need to update the notes
instead of redoing them all the time.
By redoing it I mean copying the changelog from the last time I sent
the patch and adding new entries.

My thought was that while the cover letter and series version number are currently stripped out from the start of the series, they could be added back as a supplemental commit at the end of the series (an --allow-empty commit). This could contain all of the patch subject lines and their post '---' notes as appropriate.

This sounds interesting. The only changes I can see here are the
referenced message ids, so it would be worthwhile to have the last
patch sent out first and all other patches 1..n-1 referencing the last
empty commit.
If additionally the numbering is corrected, the reader of the mailing
list would not notice any difference to the status quo, just the
sender would have the convenience to be able to track the cover letter
as an empty commit on top of a series.