UC Davis recommends solarization of the soil as a way to kill off the bacterium. More info

Here is a link that might be useful: uc davis comments on gall control

Do you have a link to a science-based source for the idea that soil pH is a big factor? I had not heard of that, though I've done some reading on gall.

I would dig as little as possible, because one thing that is well-understood is that wounded roots are likely to be infected.

I lost a rose to "crown" gall in 2010. Our soil is anything but acid. I had just done a soil test and it had a ph of 7.3. Now granted the galls were not on the roots or the crown, but rather on the cut ends of the canes higher up on the bush, still I don't think acidity had anything to do with it. I did alot of reading on it at the time and it was depressing to hear that there really was little to be done. One person said to wait 3 years and hope that the bacterium would allow another rose to be planted there without reinfection. I'm with Michael. I certainly wouldn't be digging around if there are other roots that could be subject to the activated Bacteria. On a positive note (if there is one) members of the grass family or any other monocot (such as daylilies, Iris etc.) don't seem to be susceptible to the bacterium.
Picture is of beginnings of a gall on the cut end of an infected rose cane:
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Rose crown gall happens in my garden in some acidic red clays.
That the crown gall (as opposed to the root galls) happens at the surface where the peat and other organics are tells me that even localized pH in the acidic range don't supress the galls.