When you order your pad, make sure they know what the situation is, and have your heat ranges available for them (ie, the operating range of the radiant heat, something like 60-85 degrees farenheit). The heat shouldn't affect any pad on the market would be my guess, so go with a good pad that doesn't have "memory" (crush resistant) and will last for a long time. Just because you can't see it doesn't mean you can "save money" on pad.
press ctrl-f on the main page and search for answers regarding carpet pads. there was a real good one a week or so ago.