Cool! (no pun intended)
coming from a tech's point of view , u can buy a usb fan and plug it in to the usb port in rear . the port is active, not only to network with your pc . these models have no cooling, as you are all aware, and yes, it can't breath if you sufficate it .
Cool! (no pun intended) This is a good idea, I will keep it in mind if I ever get cramped for space.
But for the poster's application, that is making the assumption that whatever he is putting on top of it leaves an adequate "gap" between the components. If there is no gap, there is no airflow regardless of any auxilliary fan; and I am sure the engineers did some basic thermal modeling, and were depending on the main cooling to come from some amount of airflow through the vents on top. This is easily solved though by putting some little self stick rubber feet on the top component (HomeDepot, Lowes, etc...).
On the same note: The HR-20's design must be fairly thermally efficient. I have mine in a very heavy cabinet, with only narrow slots cut in the back for cables. I left 4" of clearance up to the next shelf, and I leave the glass door open about 1" for airflow. After reading "Jimbos" post I decided to check the temp, it is sitting right at 127F (where it should be). I guess the engineers did a good job; but, I still would never put anything directly on top if it blocks the vents.
Take care,
Will