Why not? The Dutch had payed black soldiers from Suriname stationed in the colonies in Indonesia in the period.
To make them look more 1920's they'd look better with puttees or boots, but if you don't have problems with that, why not.
Perhaps Belgium did send askaris as an interventionist force in russia or central asia. Or perhaps a Belgian trader is trying his luck selling arms or other goods with his personal following of askaris, who he brought from the Congo where he was trading. Before his dodgy dealings where discovered by the authorities.
Or a colonel with his mercenaries.
That would even justify an armoured car!