Hey ATRflyer:
I can only speak for the US. Under 14CFR part 91 rules (Takeoff and Landing under instrument flight rules), if the pilot has been cleared for an “IFR” approach, a CAT I ILS in this case, only the ATC controller can issue a clearance for a “visual approach.” There has to be sufficient visibility from the tower controller’s perspective for that to occur. As long as the runway of intended landing is, as in your example, reported to be “RVR below CAT I minima,” I don’t believe there is any way that ATC is going to clear the pilot for “a visual approach” regardless of what the pilot says is "visible." Of course the pilot may ask for and may be cleared for a “visual approach” to land on an alternate runway – but only IF the prevailing visibility at the airport would allow it and the visibility on the alternate runway is at or above “VFR” minimums. In your example case, I don’t think the controller would agree to this because the prevailing visibility probably doesn’t exist to issue such a clearance.
If the pilot, in your example, continues the ILS approach, on the IFR clearance issued, s/he would be legal to go all the way down final approach to decision height and determine if the requirements to go below DH are present. Those requirements are 1) the aircraft is continuously in a position from which a descent to a landing on the intended runway can be made a normal rate of descent using normal maneuvers (and if operating under part 121 or part 135) that descent rate will allow touchdown to occur with the touchdown zone of the runway of intended landing; 2) the flight visibility is not less than the visibility prescribed in the standard instrument approach being used, AND 3) at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot: a) the approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable; b) the threshold; c) the threshold markings; d) the threshold lights; e) the runway end identifier lights; f) the visual approach slope indicator; g) the touchdown zone or touchdown zone markings; h) the touchdown zone lights; i) the runway or runway markings; or j) the runway lights.
If these requirements cannot be met, the pilot is obligated to miss the approach.
Hope this helps you.
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AirRabbit