On the approach chart, inside glideslope capture, there is usually a point with an altitude check. This could be an NDB, Marker beacon, DME, radial cross cut, or whatever is the specified ident for the position. This is usually a glide path v altimeter check. For an LVP approach it is also the point where an a approach ban is activated. If there is no such point then you can use 1000'agl for approach ban reasons, but not as an altimeter check. One would trust that if you are so close on finals you should not be on a false glide slope. The question to check in your Ops manual is what is the effect of a u/s OM or equivalent on making an approach.
Why is this not in 'Tech Log?'