Driven by curiosity, I took apart a cheap Chinese 7530 blower fan that I'd managed to kill earlier.
Quality of construction aside, the PCB does have provisions for a third pin, though that part of the PCB is unpopulated. The pin is labelled "O/P" and — if populated — would have been derived from FG/RD signals, all of which makes me think it's a sense output rather than a PWM input.