Well I'm not a chemical engineer mate, I've got no idea what kind of metals are used to make those rims, what the melting point is etc. Even if the melting point was a lot higher than 250C it doesn’t mean that the chemical bonds in those metals won't be weakened. I'm not sure if you'd be able to get that kind of information even from the manufacturer himself. In my opinion 250C is safe enough but I am just guessing...is it worth risking your life??

I've done a lot of car related powdercoating for my mates but mainly intercooler piping and the like. Never touched rims b4.