Anyway, the only solution is the one I described:
you have to create one single search and save it as an alert, it isn't possible to create an alert taking parameters from other panels or inputs.
I could add that the concept of alert is to have a rule that automatically checks the conditions and triggers without human intervenes.
You could also add the sendmail command to a panel, but in this way, the mail is sent every time you open the dashboard and I don't think that's acceptable.
you have only to take the search in the panel and run it in the Search dashboard, then you have to save it as an Alert, adding the other informations: trigger condition (count=0), scheduling, time frame, etc...).