Considering the objections I raised (also note that it's not all that small an area, nor that short a time frame, and that hostilities are open from the moment of declaration as far as I know), I'd suggest simply plugging the holes that are permitting abuse: Remove the ability to "assist" a neutral player as an involved player. Let people know that they're soaking up heals - this is something that more than likely isn't intentional. (I really doubt that anyone can make more of a difference by soaking up the opposing side's heals than by contributing whatever healing they can to their own side.) Admittedly, I don't have a good solution to aggro dragging on an op battle, but that's not really much different from aggro dragging in general.
Edit: I like Jayce's suggestion regarding the heal soaking problem.
Edit: I like Jayce's suggestion regarding the heal soaking problem.