I think that in T_O's case what his opponent is doing should be allowed. He is defending his flag, and if he does not double chase, then he will lose. As neither player can progress the game, it is a draw.
In a situation where a player double chases in midgame and flag defending is not involved, this should be punished.
Let's not make this so complicated. It's really simple. Double chasing is blocked whenever it occurs under the ISF rules, no exceptions. At Gravon, where double chasing is in the programming at this time, there is no "flag defense exception". If aligning with the ISF is the goal, then that means we do not differentiate between this or that example of double chasing.
Fairway, for the current Thucydides_Olorou game you say "as neither person can advance the game, it is a draw", and so in that game the double/multiple (triple) chaser shouldn't be in violation of any rule. But this thinking applies to a double/multiple chase mid-game as well. The person being double chased there cannot advance the game either, and is being blocked by the double chaser from doing so in both cases. In one case it may mean a clear loss for the double chaser to quit double/multiple chasing. In the other it may mean allowing the double chasee in to raid, pillage, and plunder a weak side. In either situation the double chaser is wanting to prevent something bad from happening to him and is double chasing to do so.
Continuing on this, take the example used earlier of a single chase. If you have one movable piece and an unblocked flag and your opponent has the Two Squares Rule advantage on you with his only movable piece, you will lose trying to block him, and that's a single chasing rule giving your opponent the win. Would anyone scream "flag defense exception" there? Hardly.
The thing that makes single chasing bad makes double chasing bad also. One person controls it and the other cannot do anything about it other than give up his piece. The rules and even the programming here on this site protect the single chasee. We are just trying to do the same as best we can for the double chasee's.