My Question:
"... there are 2x 4-pin fan headers provided, labeled CPU_FAN and SYS_FAN respectively. In the manual, CPU_FAN is stated to support PWM, however SYS_FAN is not and pin 4 is labeled as "Reserve" instead of "Speed Control". Does the SYS_FAN header support PWM or not?"
Reply from Gigabyte:
"Both the 4 pin header header does support it, however it is automatic and cannot be adjust manually. Also both will required an 4 pin fan in order to take advantage of fan speed control feature."
Apparently both headers do in fact support PWM. It would be interesting to know why the pin-4 functions are labelled differently between CPU_FAN and SYS_FAN, but I think this answer is good enough for my purposes. I live in a place where there isn't a good computer store (for components anyway) and have no choice but to order everything online. I sure would hate to have to deal with returning a pointless PWM case fan...