Telnet redirection??

You could potentially send a TCP response with some text that tells the client that no servers are available:

TCP::respond

http://devcentral.f5.com/wiki/default.aspx/iRules/tcp__respond

Here's an untested example. If you try it, could you post back with your results?

when CLIENT_ACCEPTED {

    Check if the default pool has no active members
   if {[active_members [LB::server pool]] == 0 }{

       Send a TCP response
      TCP::respond "Servers are all down.  Try later"
   }
}

Thanks, Aaron