It is indeed important to consider that different operators will approach this in different ways. One major US operator (at one time, anyway) required a missed approach if the RVR dropped below the published mins AFTER passing the marker, but only in the case of a fail-operational approach. This was operator specific; there is no FAA requirement to do so so long as the final approach point has been reached.
As far as the 757 is concerned, there is no manufacturer-specified relationship between LAND 2, LAND 3 and the minimums being used, nor is there any AFM limitation that I am aware of. The only AFM limitation states that if an autoland is required, LAND 2 or LAND 3 (as applicable) must be annunciated by 600 feet AGL.
The FAA requires that LAND 3 (fail-operational) be annunciated for approaches using any alert height or using a decision height of less than 50 feet. For CAT III approaches using a DH of 50 feet or greater, a minimum configuration of LAND 2 (fail-passive) is specified.
FAA AC 120-28D actually stipulates that, if the operator wants the ability to continue the approach following a degradation from LAND 3 to LAND 2, procedures for changing the minima from an alert height to a decision height, etc., must be specified in the operator's manuals. Most US operators interpret this to mean that a miss is required following a degradation simply because the company has no procedures specified for how to change the briefing/approach setup at that point in time.