What do you mean by control ?
An AIO pump flows at a set rate, it is not variable.
Not true. There may be some AIOs with a fixed speed pump, but many have PWM control over the pump. The ROG Ryujin series certainly does, and I think all of the ROG AIOs do (not 100% on that, as the spec sheets and manuals for them are not 100% clear on it, I just know for certain the Ryujin pumps are controllable and I've seen other AIOs that take PWM control). My Ryujin II can vary between 840 and 2800 rpm on the pump, not entirely certain what the nominal spec is for it (both fans and pumps can vary a little from their nominal spec).
On the original question, I believe ASUS normally set the AIO_PUMP and W_PUMP headers to fixed 100% by default. Check the Q-fan settings in the BIOS. It might not be configurable on all boards, that's where to look first. I can certainly adjust them on my board.