Sieve setting a flag on the server (with sieve?)

I want to run a cron job for specific users. I thought the easiest would be a sieve plugin that sets some text in managesieve.sieve. And I just have my cron job check for this setting.

This is the forward sieve:

if false # true { redirect :copy "test@test.com"; }

So maybe something like:

if false # true { # cronjob1 }

Or is there a better way of doing such a thing?