Does that legislation prohibit you from teeing off into a mercury column (sphyg)?
If you needed to get really technical, you could use a digital precision pressure meter (Druck, et al) with a data logger.
And 2 mmHg is well within acceptable limits for "monitoring", I would have thought.
I would be interested in seeing your data ... also your thoughts on how much temperature and atmospheric pressure affect NIBP testing results. My guess would be "negligible" - especially as I can't imagine either varying that much in any hospital setting.