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.
The best would have been to get the core out non-destructively and extract the broken key. 2nd best would have been to learn some hidden catch or screw I didn't see before, in case there's a next time. But yes, ultimately I did get the chain loose, and also confirm there wasn't some other way I had missed of getting to the core.