Manfield:
The use of the inner marker in this case is the standard procedure for US operators; however, it must be approved in that carrier's Op Specs. Not all carrier's programs incorporate this. If they do not, the approach is not authorized.
There was a time when a few operators that had done some modifications to their baro altimeters could fly to a 100-foot DA based on altimetry alone. But, those were "Jurassic" birds (727, DC-9, etc). I know my airline had that authorization but within a few years they installed radar altimeters.
