I fly this route several times a year and have never seen the XP equipment. I suggest you try to get 21 H or J or secondarily 17 A or B (depending on how long your legs are). 30 B and H and A and J also have tons of leg room but sometimes its a bit crowded in that area. I assume you looked at seat guru:

http://www.seatguru.com/airlines/Uni..._777-200_2.php

and seat expert:

http://www.seatexpert.com/_UA_772_Intl2.html

Bon voyage.