The thing was Ethiopia wasn't a cannon using nation yet in 1500 but neither were their neighbors in the 1400s. Then in the 1500s the Ottomans started supplying their neighbors with cannons such as in the Conquest of Abyssinia by Imam Ahmad. This led to 2/3 of Ethiopia ending up under Muslim rule during this period and lasted a short while tile Ethiopia gained the same weapons.Cannons were pretty old hat by the time the Portuguese got to Ethiopia, some being used at the start of the 100 years war in the 1330s. If Ethiopia got them in the early 1500s I'd think their enemies would be able to get them as well.
In fact it was Portugal's help that saved Ethiopia in OTL from Muslim conquest as it sent 400 soldiers and cannons and weapons to help in the Ethiopia-Adal war. My suggestion is only that they send this help sooner.
If the Ethiopians can meet them on the battlefield with equal tech from the start they can reverse the losses sooner and prevent more allowing them to take control of the whole Horn.
http://en.wikipedia.org/wiki/Ethiopian%E2%80%93Adal_War