The key broke off inside my Abus GRANIT CityChain XPlus™ 1060. A new one is $250 but a new quality padlock (went with Paclock) is $60, so I spent a while Dremeling through the lock’s case to extract the captive link. It did not reveal a way to remove the key core.
The plastic case pops off easily.
Maybe just cut one link of the chain? It will be a little bit shorter, but that might be a lot easier.