Also AFAIR, the ‘self generated GS’ is actually an aircraft referenced descent angle for FPV comparison – the GS is not earth referenced as the ILS GS is.
Well yes -- the HUD GS reference and the Flight Path Vector
are aircraft-referenced -- but their depictions also precisely conform to the planet earth.
If the HUD's 3.0-deg reference line is deep in the touchdown zone or even at the far end of the runway, the airplane is above the desired path.
The pilot need only temporarily place the FPV short of the runway until the 3.0-deg GS reference line moves "down" to the desired spot in the TDZ. Typical visual technique is to place the GS reference line on the PAPI. Once the GS reference line is on the PAPI lights, the FPV is moved back "up" to the GS reference line.
Headwind, tailwind, crosswind... doesn't matter -- the position of the FPV precisely indicates where the airplane is going.
The HUD is a wonderful piece of techology and very well suited for night-time, "black hole" approaches.