I think it comes down to a lack of flexibility with the MUA composer. If you're using an editor like vim to compose the email, it would be easy enough to read the patch into the message by running

  :r file-containing-patch

And then visually highlighting text and running:

  :'<,'>s/$/\s/

to append whitespace to lines you don't want to be hard-wrapped in clients that support format=flowed