FVWM: Question regarding actions when modifier keys are held down/released

Greetings,

I've search the website, archives, wiki, and a number of configuration
websites, and have been unable to find a way to cause an action to
happen when a modifier key is held down and/or released. I'm running
fvwm 2.6.8 on CentOS 7.

My goal is to have the following rules followed.

Rules about keys being pressed and (possibly) held.
- If Ctrl-Alt are pressed ...
  - and if no other keys are pressed within some period of time ...
    - then pop up the pager
  - and if other keys (such as arrow keys) are pressed ...
    - then execute some other action

Rules about keys being released.
- If Ctrl-Alt are released ...
    - and if the pager is displayed ...
      - then remove the pager

The use case is to have Ctrl-Alt-(up|down|left|right) change desks,
but, if one wants, then holding Ctrl-Alt before pressing the arrow key
will pop up the pager so you can see the desks to which you will be
changing.

I thought it would be an elegant alternative to keeping the pager in
the task-bar, or on the screen, but it seems that there's no way to
detect held and/or released keys in fvwm.

So, on to the questions.

1) Does anyone know of a way to get the behavior I desire within fvwm?

2) If not, does anyone know of a way to get the behavior I desire
using X11 included tools?

Please let me know if more information is needed.

Regards,
  -dave