As Hamish said, for active connections, you can just use a fallback URL on a custom HTTP profile to redirect requests to a separate host when the virtual server's pool is down. Or you could use HTTP::respond when [active_members [LB::server pool]]==0:
when HTTP_REQUEST {
Check if the current pool has no active members
if {[active_members [LB::server pool]]==0}{
Send a 200 response with some HTML and caching headers set to prevent caching of the response
HTTP::respond 200 content {maintenance} pragma no-cache cache-control no-cache
}
}
Aaron