I know this was reasonably common among southwestern corps in the mid 90s... I saw more than one corps pull bars out of coolers at a show in 1995. I've also heard of it being done by just putting ice packs (the kind you use when you injure yourself, or for putting in a cooler when you don't actually want ice) on the keys under the covers. I'd personally be a little wary of that as condensation on the ice packs will leave water on the keys. I've never actually done this personally, but I do see how, in some circumstances, it would help get better tone and more characteristic sound from the keyboard.