In '97 rode my wife's '92 XT225 for a few days in Colorado with a fatter pilot jet and the mixture screw readjusted but otherwise stock. Anytime I went over about 10,000 feet it fouled plugs and quit running. It definitely needed leaner jetting. This was in September with cooler temperatures and denser air. At the time I never thought about something as simple as the wire trick.
Short of fitting nitrous oxide to a bike--or anything with an internal combustion engine--once you get up where the air is thin you're going to lose power. At that point all you can do is reduce the amount of fuel going into the engine to compensate for the lack of air.
I read somewhere that the Luftwaffe was the first to use nitrous oxide to compensate for the lack of air at high altitude. I'm kind of surprised no one has adapted an NOS kit for this use instead of just for performance.