Okay so this is a bit backwards, but you could do the "was added to list" trigger. If this list is automatically updated through an additional smart campaign, it could work.
So basically:
1 smart campaign where the smart list is "member of smart list", the flow is "add to list"
1 smart camapaign where the trigger is "was added to list" then flow is "send email"
Also, if setting the recurrence works, you may want to do that! 🙂 Just depends on exactly what you want to do.