It's not clear if the following is contrary to the rules, but including this single simple variant opens a huge variety of strategic options to players:
Allow Player Nations to attack each others' Understandings in "third party" Nations.
Example: allow British PFs to attack Russian Understanding in Germany.
This promotes active alliances where one Nation is able to strip an 'enemy' Understanding from it's Partner, who is otherwise hobbled by that Understanding from attacking the targeted enemy's PFs elsewhere. You can see where this easily breaks the trap of rote play - even in the Historical Game.
The realism of this option is borne out by the Great War, Cold War, and today by China's methodical stripping of nations' recognition of Taiwan.
Just saw this - FOUR years later. Sorry... no where in there does it say you can't help another player in the very manner you described. Reread the rules, it's perfectly legal, no need for an optional rule.