The model I am working on assumes that Pos/neg can be modeled on an matter antimatter reaction. In this model injecting a large amount of positive energy into a negative energy being would cause one almighty bang. (which does in fact give me some ideas).
With a different set of assumptions however your concept appears perfectly valid and workable. However it would mean that undead are drawn to strong sources of positive energy like moths to a candle (again ideas run abound).