Is your home uphill or downhill relative to your pasture? You may get issues with backflow, in case it is uphill, which may clog up your valves.
Just how far of a distance are we talking about? It sounds like your second controller is within wifi range or you have run an Ethernet cable that far. Considering that standard 10/100 network uses 4 out of 8 Ethernet wires, you may use an available pair to drive a relay. If you don’t have a cable running that distance already, you can get away with a single pair sprinkler wire (more on this later).
Few days ago @DLane and I came up with a way to use dual throw relay to share pump relay between two controllers. Original discussion is here (link).
You can use a single $7 DPDT relay (like this) to control the pump relay and your house zone valve (you don’t have to explicitly have your pasture Rachio control your house valve) both would be controlled off of Master valve terminal of your home Rachio.
Note that SC terminal in this configuration connects to two terminals of the relay. Relay in this schematic is oriented in such a way that pins 7 and 8 are on the bottom.
While this is not as convenient as having the Rachio support functionality you are proposing out of the box, it may be safer (prevents internet going down in a middle of irrigation from having your pump blow out your valves).