I would also suggest a failsafe since at times x10 modules can miss commands (which, in this case, could give an erroneous indication about the fence status). Use a 2-way appliance module to power the fence. Then use AHP to poll the module to be sure of its setting. A macro would then be used to set the inside indicator on or off, using a 2-way lamp or appliance module, and it's status polled as well. It's added insurance that the inside indicator is displaying the correct fence status.
For the outside indicator, use a low-voltage light and power supply (such as garden lighting), and plug the power supply into the appliance module controlling the fence. Run the low-voltage wiring along with the present high-voltage wiring to the fence and mount the light on top. Again, the insurance is that the light mimics the fence status since it's plugged into the same module. (I'd also suggest 2 lights in case one blows out.)
If X10 occasionally neglects a light -- or turns one on for kicks -- it's no big deal and I've come to accept that as the norm. But this application could have serious consequences. Good luck!