I still haven't been able to recreate this bug [1] in mod_blog [2].
Or rather, I haven't been able to get mutt [3] to use “quoted-printable” encoding [4] I was so not expecting. All my attempts failed, and mutt sent the emails encoded as “8bit” encoded [5] (which isn't an encoding per se, more of an “indication” of what to expect).
Sigh.
In the meantime, I added a bit of code to detect the issue and abort immediately [6] so I won't have to scramble to manually fix the blog like I've done twice now.
[2] https://boston.conman.org/about/technical.html
[4] https://en.wikipedia.org/wiki/Quoted-printable
[5] http://stackoverflow.com/questions/25710599/content-transfer-encoding-7bit-or-8-bit
[6] https://github.com/spc476/mod_blog/commit/8b0193e89b9ab2dc15102b29e8f7240de78fa1b1