I use hexl-mode all the time.
I've often needed an insert, as opposed to overwrite. Do you know if
there's a way to insert?
Sadly, there isn't. I have a new mode I call "nhexl-mode" which does
"the same" as hexl-mode but in a completely different way (the
conversion is done in a font-lockish way, so the buffer's content is
not affected, only the display) so you can use it with buffer insertion
and pretty much anything else. Sadly, it bumps into some severe
performance limitations in the current redisplay code, so it's largely
unusable other than on very small buffers :-(