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!