unable to redirect to a different page on another pool

I think Chris' original example should work if the splash.html page doesn't reference any other objects on the same fully qualified domain name. HTTP_REQUEST will be triggered regardless of whether there is a pool on the virtual server or whether the pool is up.

You could also use the VS default pool instead of hardcoding poolA.

when CLIENT_ACCEPTED {

    Save the name of the VS default pool
   set default_pool [LB::server pool]
}
when HTTP_REQUEST {

   log local0. "[IP::client_addr]:[TCP::client_port]: [HTTP::method] to [HTTP::host][HTTP::uri]"
   if { [active_members $default_pool] < 1 } {
      log local0. "[IP::client_addr]:[TCP::client_port]: No members for $default_pool. Rewriting URI to /splash.html and using poolB"
      pool poolB
      HTTP::uri /splash.html
   } 
}

If that doesn't work, check the /var/log/ltm debug to see what's happening. I'd guess the issue is with content referenced in /splash.html getting rewritten to /splash.html. If that's the case, then you could move /splash.html to a new unique subdirectory and use something like this:

when CLIENT_ACCEPTED {

    Save the name of the VS default pool
   set default_pool [LB::server pool]
}
when HTTP_REQUEST {

   log local0. "[IP::client_addr]:[TCP::client_port]: [HTTP::method] to [HTTP::host][HTTP::uri]"
   if { [active_members $default_pool] < 1 || [HTTP::uri] starts_with "/maintenance/"} {
      log local0. "[IP::client_addr]:[TCP::client_port]: No members for $default_pool. Rewriting URI to /maintenance/splash.html and using poolB"
      pool poolB
      HTTP::uri "/maintenance/splash.html"
   } 
}

This latter example would work assuming any objects splash.html references are relative to the current path.

Aaron