I don't know what the logic behind not ballasting turf tires would be. You are definitely safe to put a ballast box on the three point. Tractor tires are designed to carry the tractor with a load on the hitch.
I have ballasted turf tires before. I haven't operated them for the 30 years or more that my R1 tires have been ballasted. I don't see how there could be any issues. The general point of turf tires is to run on soft or delicate surfaces, like....turf. Heavy machines will cause ruts in the soil, so maybe the logic is to minimize rutting by keeping the machine as light as possible. You can fill a turf tire with much more ballast than an R1 tire of the same height.