How to avoid juice flowing out of meatloaf when puncturing it with a thermometer?
If you have the ability to do so, cook the meatloaf with a probe or thermometer in it. That is certainly the approach I take when I have to cook a pork or poultry meatloaf or something of the like.
Otherwise, I would recommend a high angle of attack (think 30-45 degrees) from outer corner towards center of mass and you may get away with some plain-ole physics of the juices not having an efficient exit.
Good luck!