irule-based redirection & maintenance page

If your iRule really is that simple, then I have two suggestions. First, you could use variables:

 
 switch [string tolower [HTTP::host]] { 
   "www.host.com" { 
      set uppool www.host.com_80 
      set dnpool downtime_pool_80 
    } 
    "www.host2.com" { 
      set uppool www.host2.com_80 
      set dnpool downtime_pool_80 
    } 
    "www.host2.com" { 
      set uppool www.host3.com_80 
      set dnpool downtime2_pool_80 
    } 
 } 
 if { [active_members $uppool] > 0 } { 
    use pool $uppool 
 } else if { [active_members $dnpool] > 0 } { 
    use pool $dnpool 
 } else { 
     Server up LB-hosted maintenance page 
 } 
 

The second suggestion would be to replace the switch statement with datagroups. This would help if you start getting large numbers of sites and want to keep your logic separate from your configuration.