While this is far from a perfect comparison, (in fact only a generalization from a completely different study of a procedure with proven medical benefit), Canadian researchers developed an algorithm which calculates the increased risk from mammographic screening of women between age 40-55.
This study calculates the average mammographic radiation dose is 3.7 mGy and increases the risk of an incidental cancer based on annual mammographic exams age 40- 55 and every other year between 55 and 74 to 1/1000. Since there is clear medical benefit the Canadians feel this is acceptable risk.
Mammograph machines use low energy x-rays, similar to what we are told about the backscatter Nude-o-Scopes, they use low doses of radiation, similar to what we are told about the NoS, and they only irradiate the breasts (different from NoS).
Since we don't know the true details of the XR-BS machines, we can draw some rough parallels. Multiple daily irradiation to the whole body occurs for occupationally exposed populations (pilots, cabin crews, frequent flyers). Biological time to repair radiation damage is on the close order of several hours to overnight.
The risk may be higher or lower than the mammogram study, but it is a study of population exposure to radiation and a calculated risk derived.
Is this applicable? I don't know, but I think we know the general parameters of population exposure, radiobiology and incidence of cancer induction from artificial radiation exposure.