From experience, passing NAT traffic shows on TCAS as separated by whole thousands of feet. On the 767 we have three altimeters, the two mains are set to 1013 and the standby is usually a few hundred feet off even when that is set to 1013. Some operators may set regional/destination QNH on the standby, according to their respective SOP.
I'm not sure what data you are receiving, but crews are not deliberately flying the NAT with anything other that 1013 set. BTW, the Strategic Lateral Offset Procedure is not "unofficial".
Last edited by 763 jock; 17th December 2010 at 14:47.