I went to OAG Travel Planner and took a look at which dodgy carriers fly to Karachi from Hong Kong under the assumption that they might be cheap.
Sri Lankan Airlines via Colombo with a 19.5 hours lay-over is top choice in this category.
Another often overlooked option is the Dragonair flight to Dhaka, Bangladesh. There are connections available to Karachi on PIA after a 12.5 hour wait from 1am to 1:30p.m. But given that it's two different carriers, it might not be as cheap as the hazzle suggests it should be.
A better quality carrier is Qatar Airways, they fly to Karachi via Doha but the lay-over is also 19 hours.
Alby mentioned China Airlines via Beijing connecting with PIA. 18.5 hour lay-over on that one.
If you're in a hurry, CX or EK are the best choices. Couldn't see any direct PIA flights.