Yes Bob, there is a way to use any slit or pinhole test to test a flat. First you need a good sphere of well known quality. Then use the flat to fold the light beam just in front of the mirror (see below). Then you can use the appearance of the sphere in the flat to test the flat. I have done this and it works well. If the sphere is on a mount with a pivot between the tester and mirror (close to the sphere) you can set the flat at the pivot point and then power of the flat can be readily checked. The surface smoothness is obviously determined from the test itself. This is kind of a clumsy description but I think you'll get the idea.
-Rik