Well 2 solutions for you:
either use the same methode as you do while the tracking points are in the field, and then do another tracking with new points when your first points are out of sight, and then parent the null with tracked data together. Andrew Kramer have a very good tutorial about this on Video Copilot: Set Extentions
Or you can try with Mocha planar tracking, included in CS4 & CS5. It can handle this kind of problem.
Hope that helps
I'll have to give them both a try...and video copilot is briliant!