What is the environmental concern with ethylene glycol (radiator fluid)? I know it will kill animals that drink it and that it tastes sweet so they want to drink it. Other than that, what are the concerns? I can dilute a spill, or watch it until it is absorbed into the ground in order to prevent animals from drinking it. This would only happen with a major puncture and the only other time it would be an issue would be when the tire is worn out and needs changing (which would be years in my case.
The reason I ask is that antifreeze seems the easiest and cheapest route to me. I'd put enough in to prevent freezing and rust (how much?) and the rest would be water. My B-I-L also recommends some other additive, rust gaurd (?), in addition to the antifreeze. My tires are tubeless.
If a puncture and spill (of this very dilute mixture) do represent a real and serious environmental risk, then I won't do it and would lean toward using washer fluid. But getting however many buckets of washer fluid into those tires through a funnel does not sound appealing to me.