The PAPIs may be disregarded below 200 ft AGL
The CAT 1 ILS is certified to keep you clear of obstacles down to a minimum of 200ft above the threshold.
The PAPI are certified to keep you clear of obstacles down to the threshold crossing height.
Disregarding a landing aid that provides obstacle clearance and a stable approach would not be what I would call safe standard of operation.
The on-slope indication of PAPI is usually quite small. This can be expanded slightly so that the PAPI agrees with the Glide Slope down to the glide slope minimum height (lowest DH).
Everyone has to remember that the ILS reference height (the height at which the GS antenna in theory crosses the threshold is not established by measuring the position of the position in space where the signals provide an on slope indication. No. this height is derived by extending the straight line slope on final to DH until it crosses the threshold.
The PAPI provide suficient obstacle clearance for an aircraft with 3 red and 1 white to clear the obstacles. i.e. the obstacle clearance slope starts a good diatance before the threshold and is at a lesser slop[e than the approach path.
St Gallen in Switzerland LSZR is a great example fro showing these issues.
The ILS and PAPI both have 4 degree slopes. However, the ILS is set further along the runway than both the PAPI and the aiming point.
The ILS threshold crossing height is 48ft. This means that if the pilot follows an on-slope indication to DH then the extension of that trajectory followed by the GS antenna will cross the threshold at 48ft. There is a 100m displaced threshold so the GS antenna's trajectory will cross the start of the runway at 71ft.
The PAPI will cause the pilot's eye's to cross the threshold at 7m (23ft). Again the 100m displaced threshold causes the pilots eye to cross the start of the runway at 46ft.
You can see that in both cases the PAPI on-slope indication is 25ft lower than the ILS trajectory.
Normal operations and standard LDA apply when the aircraft crosses the threshold at 50ft.
If however the performance is based on short field operations then the threshold crossing height will be less.
It is clear that in the case of St Gallen, one can follow the ILS trajectory all the way down provided that the landing performance of the aircraft does not rely on short landing operations. The PAPI will indicate that the aircraft is high. The GS indications are not certified below DH but should not have any "significant" bends.
Following the PAPI indication will provide the required obstacle clearance. See the ILS 10 approach chart where it clearly requires the pilot to not fly below the PAPI indications due to obstacle. Clearly the PAPI indication is going to keep the aircraft clear of the obstacle. The PAPI are also going to combine with the Aiming point markers but not the ILS.
the angular difference between the PAPI on slope and the ILS trajectory at the threshold as viewed from the PAPI is about 4 degrees if my basic calculations are correct. To expand the PAPI on slope indication would be impossible. (That is why this is such a good example).
Following the appropriate PAPI in the approriate coverage area will always be safe (in terms of obstacle clearance).
Ignoring the PAPI could be (very) dangerous.
No safety minded operator in their right mind would ever recomend or condone normal operations that ignored a serviceable PAPI instalation as a general rule. The only exception being where the approved approach procedure fro that runway is going to have the PAPI showing high and other elemets are used to prevent unstable approaches.
To simply say something like "The PAPIs may be disregarded" would be giving the lawyers a blank cheque if someone hits an obstacle on approach.