Hi Greg,
Great to know that you have resolve this issue by your own but I would like to provide additional suggestion I would like to provide is to do "content check" in receiving string instead "200 OK".
if I understand correctly, your requirement is to monitor second site created by your web administrator which will be use for monitoring purpose. When your web administrator want to do a maintenance of actual(first) web-site they just bring down a second web-site which will help F5 to change status from UP to DOWN so they can complete their maintenance in offline state with IIS service running on it.
If my understanding is correct, you need to create customized Extended Content Verification (ECV) Monitor. Article https://support.f5.com/kb/en-us/products/lc_9_x/manuals/product/lc_config_guide_10_1/lc_monitors.html will help you sure to understand the ECV functionality.
So in your ECV monitor "Send String" should provide exact URI path of your second web-site and "Receive String" should monitor particular content available on it. i.e. word "ENABLE" should be available on web-site which will help monitor to change node status UP or DOWN.
Please ignore me if my understanding is wrong.