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?