Post-1918 is tough. Just prior to WW1, Britain and Germany had agreed in a secret treaty to force Portugal to default on her loans and trigger a sell off of her empire. The war saved Portugal from that plan. A pre-war sell off is as "easy" as delaying WW1 for a few years.
A Portugal which is much more involved in WW1 much earlier could find itself more "socially damaged" after the war. Throw in a disappointing Versailles and such a Portugal could experience revolutions and/or other political upheavals in the post-war period. Depending on what sort of Portugal comes out of that period, the empire and/or parts of it could be up for sale or transferred to the League as mandates which would then be assigned to other powers.