Looks like this question was first asked in 2002 by swanhunter without no conclusions reached:
http://www.flyertalk.com/forum/onewo...origin+lhr+lgw
And again in 2004, at which time a poster gave a definitive answer that all the coterminals listed for that city/metropolitan area would count as routing through the point of origin. No support for the answer was given. See:
http://www.flyertalk.com/forum/onewo...ty-origin.html