I'm not sure about the probability of this What If, but here it goes...
In OTL, one of the ideas considered to appease Nazi Germany was to establish multinational control over the Subsaharan African colonies. This was strongly supported by the British Left (the Labor Party advocated extending it to all colonies in 1938, and in a vote on the issue actually picked up a few Tories and Liberals), and, surprisingly,by Chamberlain, who hoped to use the colonies to reform Germany and bind it to the international system.
Chamberlain's proposal, essentially, proposed uniting the Portuguese, British, Belgian, and French colonies into a single multinational territory run by the League for the benefit of Europe. (And the natives. Honest.)
Now, I don't know enough about Belgium, but it's striking to me that the left and right were willing to consider this sort of proposal. Obviously Nazi Germany wouldn't be willing to go for this, but it strikes me that some other parties might have been willing to do so. France... Daladier was certainly a fan of colonial concessions, and this strikes me as very Popular Front-esque, but I don't know about the French Right, which generally distrusted the League.
I know little about Belgium, and Salazar's Portugal was horrified that its colonies would be sold down the river to appease Germany, but could Salazar, on his own, do anything?