iRule for maintenance page

I think a better way would be to do a check if there are pool members before you select the pool. I would look at the example:

when HTTP_REQUEST {
switch -- [string tolower [HTTP::host]] {
"www.example.com" {
    if { [active_members www.example2.com_pool] < 1 } {
         If there are no members available, generate a long Maintenance Page
        HTTP::respond ...
    } else {
    pool www.example.com_pool
}
"www.example2.com" {
    pool www.example2.com_pool
    ...
    }
}

}

Hopefully this will get you started to getting this working