This. I can't think of anything that would put them on the warpath to making an example more than this. I wouldn't be surprised if they hired assassins to kill all of the escaped slaves too, just to make a point. If people are serious about Uzkalak, well, find a way to get the Skaven to blow it up, or start favor trading for a couple armies.That would be a great way to die pointlessly, or get those slaves killed slowly and painfully just to set an example for the other slaves. Remember, these slaves would not be in good shape physically or mentally, and you'd be expecting them to escape a literal dwarf fortress city with a lot of heavily armed warriors and their constructs already in the way, ready to bust into action at a moment's notice, then somehow trek across difficult terrain with little place to hide while an elite force specifically trained, equipped, and prepared for a massive pursuit in force (with magical firepower to back it up) hunts us down with zeal. We'd then be bringing back a whole bunch of slaves, some of whom might be our sworn enemies, back to an expedition that is already short on food and very, VERY much doesn't want additional trouble for itself, let alone the forces of the capital of the Chaos Dwarves coming to wreck our shit.
With that said, if you wanted to bring down Uzkalak from the inside, tampering with the lower levels is probably the key. Step one would be figuring out what's going on down there.