david wen riccardi-zhu <dwrz@dwrz.net> writes:
>
> To recap:
> 1. If notmuch handles composition, then this is either a bug report or a
> feature request.
> 2. If notmuch does not handle composition, I'd appreciate insight from
> anyone who has managed to resolve this issue.

Notmuch uses message-mode (from gnus) to compose messages. So in
principle if you can figure out how to do what you want in Gnus, the
same settings should work in notmuch. I don't have any experience with
format=flowed, but there is some (somewhat terse) discussion in the
message-mode manual

https://www.gnu.org/software/emacs/manual/html_node/emacs-mime/Flowed-text.html#Flowed-text

Good luck,

David