I don't think you will find it written.
In final approach (wether NPA or G/S capture from above) it is definitely wrong to use OPEN DES.
As mentioned, it is written in the limitations section of FCOM3. Since it is Airbus (and also company) SOP to set missed approach altitude at GS* or FAF in the FCU selector, OPEN DES is not permitted beyond this point.
Also for the interception of glideslope from above FCTM NO-110 page 3/6 states:
"
It is vital to use V/S rather than OP DES to ensure A/THR is in speed mode rather than IDLE mode"
Regards.