I had a short opportunity to test Roland VH-12 HiHat controller with MegaDrum. Here is my findings.
1. The controller must be connected to the high impedance pedal input.
2. Controller uses the Ring for the controller power source and the Tip for the controller signal (normal pedal polarity)
3. AltIn must be set to Yes.
4. AutoLevels must be set to No.
5. I got best positional precision (around ~8-10 intermediate positions between fully open and fully closed) with LowLevel set to ~245 and HiLevel set to ~300.