Don, sharpening a belly on a slick or chisel is not difficult, just apply a little more pressure on the edges when it is laid flat on the stone. This of course will rub some the wrong way and go against their grain, such is the way when working with wood. It is not much of a belly, with a straight edge applied to the back of the slick you can see daylight at the ends, I dare not guess at the amount, but I can see it is not flat.
I try to think of a slick as being sharpened in a similar way as a single bevel hewing axe. The same effect, in the slick and the mentioned 2" chisel, is of course not as pronounced. It leaves the tool with a leading edge which is able to skew across fibers in effect easing the process. It need not be so rounded that it effects how it cuts in the bottom of the mortice.
Hand plane blades are sharpened this way as well. If one was on the ball, which I am not, they could have a number of them for different jobs and needs. One blade does not fit all jobs.
