John,
yes the CSS can do this with its probe mechanism.
You can send a true http request to the port 80 server, just like a normal user, and if we don't get response because the backend server on port 81 is not down, we will consider the port 80 server as down and we will stop sending traffic to it.
You can also querry directly the port 81 server and disable its associated port 80 when necessary.
You could also loadbalance the request to the port 81 servers....
Plenty of solutions here.
I would like also to mention that we have a new loadbalancers in the ACE c4710.
It comes with the same features as the CSS and much more.
Gilles.