I really hope the devs read this thread.
Anyway, first I thought it was because only part of Abyssinia was represented on the game map. Then I checked again and heh, it seems that practically the entire real-life Abyssinia is there, only straight-line borders on the west and south but everything is in there. So one can't really come up with a "reinforcements from distant parts of the realm" mercenary vassal. But what about making some tribes hireable?
Also, perhaps strengthening the Nubians a bit (as long as they exist) could be the solution.
One more thing I suppose one could capitalise on is the whole
Prester John legend, which the westerners were at some point crazy about, and which could be used to attract them to defend Abyssinia, provided that sufficient contact is made (i.e. something more happens in this regard than it did historically in real life), e.g. marriage, conversion (16/17th century example:
http://en.wikipedia.org/wiki/Susenyos_of_Ethiopia; I guess pulling it off during the crusading era wouldn't have been out of the whack), acquiring a common border (sort of like the Byzantine Palaiologi inherited the marquessate of Monferrato in the HRE/Italy in 14th century and that branch is actually still living) etc.
From a sorta-historical, speculative point of view, any crusade that targets Egypt would be overjoyed to be able to connect with Ethiopians, Nubians or whomever else on the south, even without a religious conversion. So perhaps a distant, remote Christian kingdom could become an ally of the entire crusade? Or even simply capable of joining in, with different dialogue if player-controlled?
Yet another common tactic in such cases in the middle ages was the "mendicant tour" manoeuvre.
The final Byzantine emperors probably spent half their lives in western courts, looking for help. So had crusader rulers before. This is totally realistic for a Latin/Catholic noble anywhere which is not Europe, for a heavily oppressed Basileus, I guess conceivable for a king of Georgia or Armenia. And, with a stretch of imagination, Abyssinia. Suppose the Abyssinian legacy runs into a king of England that's secure in his realm, has a steady peace with Scotland and France or otherwise doesn't have to worry about them, is basically sitting on 80K levies and 2000 gold and getting bored. So he DoWs the caliph, drags the entire kingdom with him to acquire the Crusader trait which makes everybody love everybody, get some piety and prestige. Alternatively, no military aid is forthcoming (like always) but, let's say, the Pope spares 1000 gold out of his 35000. Or an aging veteran crusader with 25 martial score decides he wants to go because nothing (i.e. no land) is keeping him in Europe and crusading is the only job he ever held with any luck.
Alternatively, if Byzantium overblobs, which is likely in the pre-Manzikert scenario, the Basileus probably wouldn't mind an entire king submitting as his vassal. Regrettably, I can't find the exact source to show, but Abyssinia did cooperate with the Byzantines back in the time the Byzantines were still able to make a presence at the Red Sea. This means pre-Muhammad times but also after the baptism of Abyssinia (4th century), basically has got to be 5th or 6th century. It's just a couple hundred years, the learned people would know about it, it's not like, "wow, we've discovered a new kingdom full of Christians," for the Byzantines by any means.
What else? I'm wondering about the Catholic orders. I think that just as long as they were close enough geographically (e.g. there are Christian-held counties on the other coast of the Red Sea, should be enough), they would be able to be convinced to help the Abyssinians. I have no data to back this up with but I think the Templars would be the most likely (these were the guys who ran their own diplomacy and were far from simple war mongers or opportunists, they were capable of seeing the big picture and I believe their horizons would have been broad enough to have helped Abyssinia if needed and feasible; historically it wasn't really needed first of all).
Probably a last idea but also one used by certain Christian rulers in the East: submit to the Ilkhanate. Actually, Antioch did that in the last decades of its existence, and the duke, along with his Armenian cousin, rode into the captured Baghdad. Since the Ilkhan is represented as an emperor (so is the Great Mugal of the Timurids) and the ruler of Abyssinia as only a king, I think it should be feasible. It certainly wasn't outside the mentality of the Mongols, who already in 13th century insisted that the Pope and everybody else in Europe should submit to their vassalage. And Mongol vassalage (in the case of the Ilkhanate at least) tended to mean that you were expected to make yourself useful. It didn't really rely on the Khagan being close to you and able to protect you, while, obviously, declaring war on an important Ilkhanate vassal would tend to have been a bold idea. After the dust settled, I guess that during a bad succession the larger kingdoms could obtain and defend independence, particularly if they allied each other or somebody else.
Anybody feel free to poke me if you'd like me to back up some of the claims.