Frank, I stand corrected. I went and looked it up and, low and behold, they do have exhaust valves.
Here's an article on it. It doesn't seem like it would be too hard to manufacture one with no exhaust valves (basically it would look just like your weed eater block/head) as long as it had enough boost.
Just imagine making one of these with a super-turbo charger (to get rid of the supercharger load at above idle) and some high performance lightweight pistons/connecting rods. Diesel with a 9k RPM redline!