Are there any plugins for Geany / Gedit to support reading binary/hex files?
Perhaps you would be better off using a dedicated software. I confess that I have not used it but Bless looks interesting, 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!
There is a simple trick in Geany I use for easy switching between Geany and GHex.
Activate the built-in file browser plugin in Geany and go to its preferences to configure it. Check the option “Follow the path of the current file” and set an external open command, e. g.
ghex -g 1024x768 "%f".
Now open any file with Geany. If you want work on the hex code simply go into the built-in file browser (it's always in the right path) and use right click on your file and select “Open externally”. Geany will open the file in GHex automatically. After you are done with GHex, close it. If you have something changed in the file you just have to reload the file in Geany. It's a simple efficient workflow.