The balloon analogy would work regardless of where the balloon is, because you would be constantly referencing the 'walls' of the balloon to determine the center point. Even if the balloon is constantly expanding in every direction the rate of expansion along any given axis should be consistent, even if only for that axis. If you can measure the rate of change you can work your way to a common midpoint- Edit:-The experiment does not rely on any celestial body other than the CMBR, which can be measured and mapped. If it can be measured and mapped, then a midpoint can be found. EVEN IF that mid-point is incorrect, it will be absolutely vital for this to be done prior to mankind ever having a hope of traveling outside of the solar system.(Even if that is a century down the road)