Hi Josh, you should be able to configure a GTM pool for each datacenter, containing the RPC client access virtual servers you've set up for that datacenter. The GTM will consume the LTM monitors for each virtual, but you can also create GTM TCP monitors for each service and apply them to the GTM pool. If you select advanced config under the pool properties, you can set the availability requirements for that pool by choosing the number of monitors that need to be up to consider that pool healthy. You can then create topology records to make decisions on where to send clients (you'd need to choose topology as your LB method of course).
That said, is there a reason you're using RPC client access rather than OutlookAnywhere? With the HTTP-based Exchange services, the BIG-IP can do better monitoring, offload SSL, and use APM to secure access to the virtuals.
Nimbostratus