The balloons likely already have batteries and GPS receivers. An extra radio and antenna would add a few grams of weight and tens of mW of power consumption.

The balloons likely already have batteries and GPS receivers. An extra radio and antenna would add a few grams of weight and tens of mW of power consumption. That could be well under one percent more weight. It's much, much lighter than a radar reflector, which aircraft weather radar displays aren't even designed to display.