citizen_elah is correct. The connection limits only apply to the NEW connections that do not have a session persistence entry.
Using a different metric other than the number of connections is tricky because it's tough to inject metrics into the data-plane for LB decisions. You might be able to do this with dynamic ratio load-balancing. That is, you should be able to set the node ratio to zero if its max load is exceeded. If that is the case, the node won't be selected for new connections, but persistent connections will continue to use it. Note that this is different than marking the node down. You can direct traffic to a sorry page using either an LB_FAILED rule or an HTTP fallback host.
Nimbostratus