Pierre,
DA- Field elevation = Ceiling, e.g the Height AGL you will be at the DA or MDA. If you set this on the RA, you WILL get a "Meaningful Warning" although, as has been stated, it will be reference to the ground underneath you, not the threshold elevation.
HOWEVER, the actual, legal definition of DA and MDA is that they are an ALTITUDE, which is a height above MSL as determined by an Altimeter set to QNH.
Cat II and III approaches have DHs (Descision HEIGHTS) which are heights above the ground as determined by a radio altimeter. As such, the groung beneath the aircraft at the DH point is surveyed and taken into account when determining DH.