OzEx - nothing to do with the ILS set-up. Imagine a runway on top of a cliff. Half a mile out you could be at 500' agl. Two hundred yards later you could be 20' agl and all of a sudden your decision height is GONE because the pilot was relying on RadAlt rather than Baro for a Cat I approach and didn't read the notes on the bottom of the plate properly.
There is nothing wrong with using RadAlt from about 100' down if you don't get an automated readout, but as a guide only. Setting a RadAlt DH (particularly single-pilot) and relying on it is a very suspect practice. Cat I approaches are by definition by sole reference to Baro altitude.