Interesting thread. I am a spinal surgeon, and do not believe there is any evidence that pressure alone has any negative effect on bone proliferation.
Other factors, however, might. For instance, scar tissue at the operative site might (due to altered blood flow or vascular permeability) have different nitrogen loading and off-gassing properties than native tissues. Avoiding high nitrogen loading has been advised by DAN for the first 6-12 months.
The best reason for caution when diving after spine surgery in my opinion would be to avoid re-injury from the actual physical activity.