I'm curious why you think you need to have an "appropriate" DOF when positioning an ND Grad filter? I use ND grad filters all the time, and I've never needed to see the correct DOF to position them correctly. ND Grad filters are primarily contrast-balancing filters, and are designed to reduce the contrast differential in scenes with higher dynamic range than you can take with a single shot. Unless you are using a grad filter with a VERY hard edge, positioning is a rough process, not a precise one.
I wouldn't worry about using the DOF preview button to position your filter. Either with the viewfinder, or with Live View (I personally use live view, as it is a far more effective tool than the viewfinder, IMO), set your exposure (aperture, ISO, and shutter speed), and just slide the filter up or down WHILE metering, and leave it in place when your meter centers (or points to the right EV for the kind of shot you are taking.)