That's doable.
I don't have any idle triggers, so I'm not going to take the time to pony one up.
Here's one example:
Trigger: when Mac is idle (THEN STOP) - #6 by _jims
I'd probably:
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Use a global time-of-day triggered macro set to 2:00pm to activate an IDLE-Trigger sub-macro.
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If the IDLE-Trigger macro activates Messenger then it disables itself and won't be activated again until the next clock cycle.
Keyboard Maestro has IDLE, Time-Of-Day, and Cron triggers amongst others: Macro Triggers user manual section.
So you have a great deal of flexibility.
-Chris