The Elk will work fine with RCS zone controllers installed. It just doesn't talk to them. I know, sounds like a funny statement but here's what I mean:
1) The RCS thermostats and zone controllers all operate on the RS485 network.
2) The Elk talks on this RS485 network but talks directly to each thermostat.
3) After the Elk gives the commands to the thermostats directly, the thermostats then communicate to the zone controllers.
This is exactly how we installed the Elk with the RCS thermostats and the zone controllers in the North Texas HA Users group project house. In that house, there are 10 thermostats and 3 zone controllers. Everything works great. If you think about it, there really isn't any reason why the Elk has to talk directly with the zone controllers.
A cable was installed from each zone controller to the Elk. Each thermostat is then connected to the appropriate zone controller. The zone controller acts as a 485 Hub for the thermostats and the Elk to communicate with each other. Once unique addresses are all set for each device the Elk will be able to communicate with the thermostats. The thermostats actually communicate with the zone controller to control the dampers and the actual HVAC units.