Are there any plugins for Geany / Gedit to support reading binary/hex files?
Perhaps you would be better off using a dedicated software. I confgess that I have not used it but the softwarte below looks interesting:
https://apps.ubuntu.com/cat/applications/natty/bless/ as mentioned by the comment above (just saw it now..)
I've written a script for the Geany Lua plugin that allows you to do hex editing.
The file is available from https://wiki.geany.org/config/scripts/lua
You just have to install the Geany Lua plugin (available via apt-get as geany-plugin-lua), copy the script into your config directory (~/.config/geany/plugins/geanylua), and restart Geany. The script will appear under Tools - Lua Scripts.
If you have the Bless hex editor, the script will open files in that. Otherwise, if you have xxd (which comes standard with vim), then it will allow you to edit the file as hex within Geany.
Feedback is welcome!