I'm not sure what you mean by "^"+"u". Do you mean Control+u, or the actual ^ key that is right next to the P key on an azerty keyboard? If it's the latter, I don't think it's possible. You can create key combinations only with modifier keys, not "normal" keys.
If you need the Control key, you probably first need to remap it to something else, like mode_switch. It would be much easier if you used the AltGr key as modifier. I can remap the U key in my keyboard (keycode 30) to output the ũ and Ũ characters using AltGr+u and Shift+AltGr+u, respectively with:
Code: Select all
xmodmap -e "keycode 30 = u U Greek_theta Greek_THETA utilde Utilde"
(I also need the greek θ and Θ characters for my greek keyboard)
I don't know the keysym for the esperanto accented u, but if you do something similar for the keycode you want to use and use that keysym instead of utilde, it should work with the AltGr key in a similar way.