Like most questions, the FAA lawyers have posted their answer. This is from a letter in 1999.
"You first ask whether it would be proper under FAR 61.51(g) for a properly qualified SIC to log instrument flight time flown during instrument conditions while serving as the SIC in Part 121 operations on an aircraft that requires two crewmembers. The answer is yes. As a qualified SIC, and as a required crewmember, you are "operating" the aircraft within the meaning of FAR 61.51(g). Therefore, as the SIC operating the aircraft "solely by reference to instruments under actual or simulated instrument flight conditions," you would log that time as SIC flown in instrument conditions. Naturally, the PIC logs the time as PIC flown in instrument conditions.
You then ask if, for the purposes of maintaining instrument currency, an instrument approach on the above flight flown by the PIC can be logged as an instrument approach by the SIC. *The answer is no. As the SIC you have not "performed" the approach as contemplated by FAR 61.57(c) because you were not the sole manipulator of the controls during the approach."