Switching Classes on ASM based on pool availability

Your iRule should always take priority so I don't see a problem with having two classes (or none for that matter) attached to your VS. If you don't want to use a default pool, you should be able to specify the pool within the iRule as well.

Using what you have above: 
  
  
 when HTTP_REQUEST { 
     if { [active_members [LB::server pool]] } { < 1  
         HTTP::class select $anotherHttpClass 
        pool server pool } 
 else { 
     HTTP::class select $anotherHttpClass 
    pool another pool 
     } 
 } 

Obviously spaces aren't compatible, but you get the idea.