Can the touchdown RVR be replaced by the midpoint or rollout RVR?
I'll start by pointing out from Wikipedia that—


  [The RVR] values obtained are not a reliable guide to what a pilot can actually expect to see. This can easily be demonstrated by the fact that when obscuration such as fog is variable, different values can apply simultaneously at the same physical point.


They give an example for that.



The answer to your question is yes it's possible to use the midpoint RVR if the touchdown RVR is inoperative. But, authorization is required, and it cannot be used for ILS CAT II and III.


  If the touchdown RVR VS is inoperative, some operators may be authorized, via OpSpecs, MSpecs, or an LOA, to substitute an operative mid-point RVR VS for visibility minima greater than or equal to 1,800 RVR.


Source: FAA 6560.10C