didn't realise you wore hotpants- what next? a village people CD for christmas?
You could look at the problem the otherway and keep the heat away from your pants rather than making your pants heatproof. I know that Neil put a plate on his bike to stop the heat from the exhaust. DRC do a universal heat protector for header pipes 17.99 on fleabay.
I have some ally plate and might be able to make something to fit if it helps.