I need to write this up and submit it for an article I suppose.
The cheapest and easiest way I know to put liquid ballast like washer fluid or similar is to buy a cheap one or two gallon sprayer from some place like Home Depot. Don't install the hand wand on the sprayer, but complete all other assembly steps. Tires can be filled while still on the tractor, but use a jack to lift the tire off the ground and take the weight off the tire. Remove the valve core from the valve stem, allow all the air to blow out. Push the rubber sprayer hose over the valve stem, it should fit snugly over the valve stem. Now add your liquid to the sprayer, and pressurize the sprayer using the hand pump. Keep an eye on the liquid level and pump up the pressure again when the liquid is halfway down. When the liquid is all gone, carefully release the pressure from the sprayer, which will also allow the tire to "burp" all its air as well. Repeat as needed to fill the tire. Once the tire is full, when it is "burped" the excess fluid will drain into and be caught by the sprayer. Alternatively, once no more air can "burp" out of the tire, a final gallon can be added and forced into the tire for maximum weight. Some air will remain trapped in the tire to provide cushioning. To finish the job, pull the sprayer hose off the valve stem and re-install the valve core.