I've been playing around another idea:
To manually set the secondary IP Address of each machine as a Custom property, and then use it in the HTTP Monitor on my template.
It's still very "manual", but saves the time it takes to drill into the HTTP Monitor inside the template of each node and bypass the ${IP} macro.
However - I can't seem to get SAM's HTTP Monitor to use this custom property as a macro variable.
Am I missing something, or is it simply not supported?