There are two sides to this:
1.) Soviet susceptivility to attack in case of no M-R pact:
As far as I know this behaviour is already coded, either via vanilla or via CORE.
2.) German deployment / Barbarossa Ai:
As far as I know this is a potential problem, since there are very refined events that deploy Germany along the M-R line and attack Russia when deployment is sufficient, but none that cause similar behaviour along the former Polish/Soviet border.
That´s actually the first thing I have to check when I come around to doing this.