Did a bit of thinking about this.
Aircraft operating on airspace whose ATC uses inHg would need a conversion table, as atc instructions would all be in inHg, and would be responsibility of an aircraft's crew to do the conversions.
I would guess NATO countries with legacy soviet equipment probably already use those.
I made a table, but apparently there is some other bug that affects the accuracy of the pressure settings (http://forums.eagle.ru/showthread.php?t=130908&page=2) pressure_conversions-1.0.0-inHg-to-mmHg.pdf
