Hi Matt,
Since most Web Services do not have long connection times I find that Round Robin or Least Connections (member or node) generally suffice for normal operation.
If you truly want to create a custom type of monitor you could create a custom Health Monitor to put on the nodes that would read your endpoint and based upon the numeric value, remove the node from the pool of available servers (probably undesireable since you could create a situation where you have no members in the pool and things start to fail).
The other load balancing methods use thier own built in methods for determining which server to send the traffic to:
Dynamic Ratio (node) : This method is similar to Ratio (node) mode, except that weights are based on continuous monitoring of the servers and are therefore continually changing. This is a dynamic load balancing method, distributing connections based on various aspects of real-time server performance analysis, such as the number of current connections per node or the fastest node response time.
Dynamic Ratio (member): This method is similar to Ratio (node) mode, except that weights are based on continuous monitoring of the servers and are therefore continually changing. This is a dynamic load balancing method, distributing connections based on various aspects of real-time server performance analysis, such as the number of current connections per node or the fastest node response time.
I've looked at but not implemented the SNMP Monitor myself, but the method will again remove the node from the pool if the conditions you set are experienced.
Hope this helps.