Stopping water from bubbling over when cooking rice
You've stated that you're not washing the rice.  That's the reason this is happening.

Water boils over because of starch.  Many types of rice (brown rice included) can be very starchy, and this could conceivably cause the water to boil over depending on the amount of rice/water and the size of the pan (or rice cooker).

Washing the rice also helps to prevent the grains from sticking together, so you would want to do this anyway (unless you are actually trying to make sticky rice).

It's very simple to wash the rice; just keep adding and straining out cold water until the water runs clear.  You'll probably be surprised at how much starch actually washes off.  If you do this, your water should stop boiling over - regardless of temperature.