I managed to find some HMC5883L modules on ebay for <$1 US shipped, so I picked some up and now I just have to wait for them to arrive from overseas. If the tests go well, then I'm hoping to be able to desolder the components from one of the boards to use on the final PCB, which is why I ordered a few. For the brains of the device, I'm considering using an ATTiny85, as its the one of the smallest microcontrollers I have on hand and it has just enough pins to cover the I2C to the sensor and LEDs (2 LEDs/pin). I plan on testing the concept at some point, but ultimately I'm waiting on the sensors to arrive at this point.
EDIT: I also did a quick mockup of what it may look like (using Eagle and SketchUp) just to get rid of the blank image for the project.