there is no direct way of doing this.
However, my solution is to do this :
service sorry_down
....
service sorry_overloaded
keepalive type script check_service_down use-output
...
owner mycompany
content www
vip ...
add service ...
primarysorryserver sorry_overloaded
secondarysorryserver sorry_down
active
The script check_service_down, will do a 'show service ' grep -u Alive to detect if a service is alive or just not used because down.
Or you could also simply do ap-kal-pinglist and ping the services.
Anyway, the idea for the kal for the service sorry_overloaded is to check the status of the other services and detect if they are down or just overloaded.
Gilles.