OP says that the letter says it "questions whether it is even safe to stand near an operating scanner, let alone inside one."
Um, helps to read the fine linked document, which has been partially redacted, but still says "Individual effective dose per screening (frontal and rear) of a subject is , less than the 10 urem (0.10 uSv) limit. Further down a standard (NCRP 1993) is quoted which "recommends that members of the general public receive less than 1 mSv (0.1 rem) per year."
So, if these numbers are compared (who knows if they are reproducible) you are considered safe up to about 10,000 scans per year (1 mSv / 0.10 uSv).
The document does indicate there is a potential danger from X-ray beam overshoot "above and behind" the scanner. Yes, but note in the diagram this area BEGINS at 13.8 FEET above the ground, and RISES IN A CONE!!! So, you may be at risk if you're about 14 feet tall (or work in an office on the second floor?) standing behind the machine...