I'm wondering if it's possible to forward slip a large commercial aircraft, like the A320 or even B777, and if so: is effective or useful, since most small aircraft which I gather do it have no spoilers for instance which large aircraft do?
It's it possible? Yes, you would do it the same way as in any other airplane.
Is it effective? Yes, just look at all of the fuselage area that is exposed to the relative wind.
Is it useful? Not really. With all of the other drag devices, you shouldn't really need it.
Is it recommended? You would have to check the AFM. I looked in the A320 Flight Crew Training Manual and there is no limitation against it, but if you need to slip then that's a strong indication of poor planning and an unstabilized approach which would require a go around. Even at higher altitudes, it isn't very comfortable for the passengers!
It’s been done before by a large craft with engine trouble, a 767 operating Air Canada flight 143. During the change over from imperial to metric there was a flub on calculating the amount of fuel in the tanks of the craft by the ground and cockpit. A tagged out digital fuel gage was the perfect setup for running out of fuel. The captain was an experienced glider pilot and knew how to slip an aircraft and even though it was risky to punish the frame of the aircraft there was little choice. They were able to land safely minus only the front gear on a old airport turned into a drag strip.