I prefer it in its current settings, though why not include a way to use the old method? even if through a registry option.
If only you had a way to detect which monitor is which, then a setting to 'always open metro on X, option would be feasible completely solving his problem?
Heres a potential idea for detecting screen order, since it seems you can only get a temporary ID that may change after a system restart.
1. when the classic shell first starts up, maybe you could use a 'couple pixel' start button in the bottom corner of each screen, each with a unique color combination:
then parse a screenshot to determine which monitor belongs where.
http://stackoverflow.com/questions/14110102/getting-screenshot-and-find-location-of-multiple-smaller-images-in-it-in-linux