Is there a way to capture a release of the modkey using e.g. the event hooks?
I've tried modifying the source for debugkeyevents:
http://xmonad.org/xmonad-docs/xmonad-contrib/XMonad-Hooks-DebugKeyEvents.html
but this only captures the release of "proper" keys, not the modifiers. (E.g.
pressing mod+j will generate a press and release of the j key (with
information about it's modifiers), but not the press and release of the
modkey).
This is because there are no grabs established on the bare modifiers, nor is there a generic "send all key events" mask in place, so X11 doesn't bother to send events for them. If it sent an event for every little thing that happened, unsolicited, most programs would be busy-looping on noise events instead of doing useful work; as such, you should always remember to *request* the events you want.
So in this case you probably want to use grabKey (aka XGrabKey()) to select for interest in the modifier key by itself; the handleEventHook handler should ungrab the keyboard (you don't *really* want xmonad to seize ownership of the keyboard every time you touch the key), do whatever you want to do, and pass the event on.
(This is largely what the other suggestion does, once you poke at the code.)