Well, it's certainly not a big problem to do the NA and Euro centers; simply define some bounding box of lat/long to select said points, repeat calculations, and have xplanet center appropriately.
XP weighting might be interesting; it's not as easy as the .xml file I get from jcwren doesn't have that info in it, but I'm sure there's ways of getting it in a couple more steps. I'd take Merlyn & vroom out specific only to avoid such problems. Fortunately, the method of generating this stuff is simple once you have the data.
Dr. Michael K. Neylon - email@example.com
"You've left the lens cap of your mind on again, Pinky" - The Brain