Not sure what you mean, but ML bracketing can shoot at different ISOs, and if it's too dark, it goes to Bulb mode.
Offtopic: I've been experimenting with exposure simulation in Bulb mode, based on bulb timer setting. On new cameras it has a lot of potential, since the sensors have little noise. For example, 5D3 can get huge ISOs in LiveView, unlike 5D2. And when it's too dark outside, you can enable FPS override to get a cleaner image, and ML exposure simulation will be still correct.