The action around Kassala is interesting, but not much written about. Kassala is one of the most important centres for Sufism in Sudan. The ruling Khatmiya brotherhood had close religious links with Egypt, and was garrisoned by Egyptian troops - the city fell to the Mahdists in July 1885, six months after the fall of Khartoum. Apparently the siege of Kassala, nearby Sennar (on the Blue Nile), and Gallabat (on the Ethiopian border) and Egyptian retreat with refugees to Massawa (Eritrea) were all quite dramatic, but I can't find out much about them except a reference to an Egyptian officer called Saad Rifat who organised the fighting retreat and rescue with much heroism.
The Italians occupied Kassala again in 1940, but that's another story...
As for Mahdists against Ethiopians, there was plenty of action because the Sudanese were pursuing an active policy of jihad to expand their borders. Lots of action and border raids around Gallabat. At one point in 1887, the Mahdist army even turned on itself, when the local commander claimed to be Jesus (also a prophet in Islam), whose second coming was meant to follow the earthly return of the Mahdi!
Control of the border ebbed and flowed - there was a big Mahdist push into Ethiopian in 1888 - but things were finally settled at the Battle of Gallabat in March 1889. The Ethiopian king Johannes (John) IV personally took control of his army and was killed, along with an estimated 15,000 others. The Mahdists held the day, and took so many captives back to Khartoum that they caused the prices in the slave markets to collapse. Unfortunately for the Mahdists however, the battle also broke the cream of their army, and effectively put paid to any expansionist plans. From that point on, all their military activities concentrated on defending territory, not trying to expand it.
Got to be some games potential there, especially with all those lovely Abyssian 28s you can get these days!