If you don't mind them being not
quite right, then yes, I think German Askari would make a good proxy figure for the late war khaki uniform.
I think you could probably get away with using a British officer in a Wolseley sun helmet. The collar on the jacket was different, but otherwise they were very similar.
This photo is from 1940, the uniform hasn't changed much since 1918, but they now have a collar just like the British one, whereas in the Great War it had a high collar.
So yes, I think you can get away with using a British officer. They also wore peaked caps, virtually identical to the ones worn by the British in WW1.
I must admit I have almost no knowledge of Force Publique uniforms between the wars, I am basing my comments on late WW1 styles. I have seen a photograph, purporting to be from the late 1920s, which appears to show them back in the pre-war blue zoauve-like uniform (with excessively baggy pants). But the dating of the photo may be incorrect, I can't be sure.