How would you want this to work? "Last Updated" is more like an attribute (though not technically one--more just part of the device metadata). There's not a command you can run, which is what a custom action would accomplish. It sounds like you would more want it to be usable as a trigger or possibly in a conditional, either way acting as a sort of pseudo-attribute whose value you can read (and do with as you please: compare, wait for it to change, etc.).
That being said, for the purpose I think you're aiming for, I'd just recommend Device Watchdog.
To use in calculating elapsed time since last update. My research on the topic was a fail and completely my own fault. I have completed install of Device watchdog. It is doing exactly what I needed. Thank you.