Wow those look preatty awesome. you should definetly mount the servo and reciever under the car. I think its futaba who makes a super thin servo. I saw it in a magazine once. Should have plenty of torque, and if you mount it on the bottom with only a small shiny metal screw head or 2 poking through the floor boards, it wouldnt mess up the look, and becuase it isnt very fast its not like you need to worry so much about messing up the servo, or reciever with all those crazy jumps your gonna be taking it off . Ive got the plans all drawn out in my mind. and if there were moving parts on the bottom like a driveshaft you could take a sheet of lexan, and make a cover so you could seperate your servo, and reciever from the moving parts. You wouldnt be able to control speed with this set-up, but it looks like this thing only has 1 speed anyway

Im assuming you already have one, but if you wanna do this, you better hurry. I found a great deal on the le mans one. On the companies site you linked 2 it was one of the most expenive. On the link below its selling for nearly 1/2 the price listed on the site. On the site they said it should sell for $460, its selling here for $250. It is a pre-order, so your gonna probalby wait a bit to get it, but its worth it. Im very very tempted to do it.
[link=http://www.ministeam.com/acatalog/Mamod_Steam_Cars.html]Its around the middle of the page[/link]