B737900er, it's not described in the FCTM. In my opinion it is not within the responsibility of Boeing either, but the airline to make a decision of the appropriate method, given they may operate under different regulations or set higher requirements than what is regulated by the authority.
As I wrote previously, e.g. my airline decided to raise the circling minima to 3000 meters visibility given the fast speed of the -800. Hence, min visibility and timing are interconnected, and at my airline it has been considered as show in the calculation in my post on page 1 of this thread (before the discussion was derailed by AirRabbit). Another airline could in theory decide on 5 km min visibility and describe the circling as a visual approach if so inclined.
I think most of us would agree that a visual approach is flow looking out the window, with the only necessary working instrument being the airspeed indicator.