Trigger message to client based on health monitor response from server

An irule cannot see the status response of the health monitor - it can only see if the pool members are up or down.

If all your pool members are down, you can use the http profile fallback host setting to send the client to another address with a maintenance page.

You can also set the "fallback on error code", but your monitor will need to mark the pool member as UP for a 5xx response as well as a 200 response.

K7124: Configuring the BIG-IP LTM to redirect client requests based on HTTP server response codes

Please note: using "Fallback on Error Code" prevents re-balancing connections on pool member down. This may not be desirable.

I hope this helps.