This link shows that Belgium was actually not very pro-Allied:
http://worldwar2database.com/html/belgium.htm
Belgium wanted to remain neutral and so did Holland. It was only when they were attacked they reluctantly accepted the other side to join their territory. So if Germany turned east instead of west then I believe it's very unlikely that Belgium would join the Allies.
At the same time the Allies didn't have the will or firepower to wage a war in 1939 against a neutral country. A very early Allied aggression against neutral countries like Belgium and Holland would certainly be seen in USA as a war of aggression and not in the interest to protect against Germany. So we could e. g. do the following.
The USLL destroyed will NOT be given to the UK in March 1941 because the USA looks upon the Allies as aggressors too. Allied convoy sizes would be e. g. 25% lower until USA enters the war. This because it would be harder to generate as much supplies for UK in USA if the US public was more hostile towards the Allies.
By doing this we don't prevent the Allies from doing what they want, but it has a consequence. They gain an early benefit of not losing efficiency in the west, but they pay the long term PP price for some time. Germany can then move on to blitzkrieg because there is no buffer between France and Germany.
Please vote on each of the following:
1. Allied DoW upon Belgium or Holland will not spawn the USLL destroyer
2. Allied DoW upon Belgium or Holland will lower the convoy size by:
a. 5%
b. 10%
c. 15%
d. 20%
e. 25%