You probably know, toggling case (lower vs. upper case) can already be done with `Ctrl`-`Alt`-`u` and this shortcut can also be configured.
For CamelCase conversion, ages ago I wrote a simple little Python script to just do that: https://gist.github.com/eht16/52067a31f2d8cfc3c0fac87d2ab70e28
Configure this script in Geany as command in the "Send selection to" settings, like in the screenshot. Then select some text and use `Edit->Send Selection to->CamelCase`. The script also tries to detect if it is CamelCase to convert to lower_case and vice versa. For me this works fine since a couple of years.
If I find time, I'll put it in the wiki. HTH. 