A reflection target can only send a limited range of illumination back to the camera - something like 3% to 95% reflection. A transmission scale can send a much wider range of light because you are shooting the light through logarithmically small dark patches that can be a hundredth of a percent or less. With reflection, you get a few percent of the bright light in your darkest patch.
Testing using a reflection target is possible, but you tackle the problem with a different approach. You take several shots per test series shooting something like a gray card, altering the amount of light hitting the film - one step at a time - by varying the shutter speed and f/stop. It's a lot more work for full film curve testing, but good for speed point tests.
Sekonic makes a reflection target design with an 18% gray patch closely surrounding with patches reflecting plus and minus 1/6, 2/6, 3/6 stop. You might be able to find speed point faster with this target than you would with a standard gray card.