That's a no then isn't it? If you can only think of 1 that is no where near the scale of the ones in ck2 that's a no. And you didn't answer the question about the Hammers army.
Scale of the ones in CK2? Do I need to post the patch notes for the beta patch again? They scale from small to huge. And those are just the ones I could think of off the top of my head right then.
You could consider Anglo-Saxon response to the invasion(s) of 865 a coalition against the Vikings. The
mycel heathen here was not just a single prepared invasion or war, but closer to a series of smaller wars that results in a partition of England between the Anglo-Saxons and the Scandinavians. If it was a single war like a prepared invasion in CK2, there wouldn't have been a partition so much as a either a complete defeat of the Vikings or the annexation of a single Anglo-Saxon petty kingdom. (The Vikings got portions of Mercia and Northumbria, independent realms at this point, so it's not just a single war. That's why the game portrays the 865 start the way it does.)
The initial response to the Mongol invasions of territory of the Cumans resulted in the first contact between the Rus and Mongols before the actual invasion of Rus territory. (Battle of Kalka River) The principalities were not really unified as a single state at the time.
The further subjugation of the states in Rus resulted in a bunch of fighting that might as well be a coalition. Of course, once these territories were subjugated, they still existed but could no longer join coalitions against the Mongols. (The Mongol attacks on Poland, Silesia, and others represent a good use of simultaneous war declarations to get around defensive pacts.)
I'm not going to continue listing instances, as I don't have the time. I considered looking at Mongol conquests, but those didn't matter in the end (the Mongols just crushed the opposition regardless). Which, I think still fits in the game mechanics.
Regarding the army at Tours:
From the information we have available (there's a lot of missing info), if we do not count the mercenaries from pagan realms, the Christian army at Tours included armies from territories controlled by the Merovingians, but also areas that they did not own at the time, including Lombards from Italy.
One problem with this discussion is that CK2 does not really deal well with the ambiguities of fealty, homage, and diplomacy in the period. Don't get me wrong: it's better than just about any game out there. But when trying to determine whether the game is modeling things correctly, we have to make choices like "Should we model the HRE as a single state or a coalition of smaller states in a kind of confederacy? How do we model invasions like Viking attacks on England?" and so on. The defensive pacts seem to be a way to help model yet another weird in-between aspect of diplomacy and domestic policies in the period.
I still think that tying it to the rest of the diplomacy system would be better.