My Mom has pretty much the same board that we got from my great-aunt (dates from the 40s, I'm sure). I'm getting the board when we finish remodeling (and I get my dedicated quilting room back!). The reason why they used layers of regular fabric in the past is that it enabled you to press clothing properly and get sharp edges. If you are quilting, it's easier for YOU to have a little give. Your plan seems reasonable.
Ok.. it has muslin against the wood, a layer of fairly thick cotton batting, 2 layers of a thick flannel (almost like a thin blanket) and then a cover. So, I think using that layer of thinsulate will cover that 2 layers of thick flannel which I would guess is protecting the board from the steam to prevent warping.