I work as an Apple hardware service technician.
The boards are reworked, and that includes devices that have been subject to liquid damage. The damaged assembly is ultrasonic cleaned, then damaged components are removed and replaced until the circuit passes a functional test. Boards with damaged pads or traces don't typically end up being recirculated back into the service pool, however in some rare instances a board with a jumper or lifted pad that is considered non-critical, such as a pad for a component that was not included in the final design (connectors used during prototyping and so on), is considered acceptable for use.
I somehow doubt a board with widespread liquid damage would be repaired as the number of components requiring replacement would be rather high and the labour would be extensive, but boards with localised or isolated liquid damage would certainly be considered serviceable, and it makes more sense to perform some repair to an existing board than manufacture an entirely new one for a machine that may already be several years old.
Not to suggest this is somehow a scandal and cause for alarm however. I don't believe the failure or dead-on-arrival rate of these reworked liquid damaged boards to be any different from normal failure rates for refurbished boards.
Whether it would be considered acceptable practice by a qualified electrical engineer, I wouldn't know. This is simply an observation from someone on the front lines, turning the screws and sending machines back into service. It does happen, those boards are refurbished and used as service parts, but whether it's a good or bad thing is up to others to decide.
On the discussion topic at hand, using rice to remove moisture from electronics is an exercise in futility. While the water is removed, the minerals suspended within the water settle on the board. Corrosion sets in as metals on the board start to react with the water and suspended particles, a process accelerated when a current is passed through the board. I've retrieved data from and was able to successfully power on a machine that was submerged in a kitchen sink for 45 minutes, and that machine was still dripping when it came in. Skip the rice, get it somewhere quickly to have the power source removed (battery disconnected) and the board properly cleaned. The chances of reviving the system will be much higher.
