Had a look at PANS-OPS ICAO 8168, there is no OCA/H adjustment for this situation shown in there so the 'adjustment' must be a non specific obstacle height to be used in the instrument procedure design it seems.
Had a look at CAP232, suprisingly, there is no specific reference to including holding points and the categories of aircraft that might use them into the airfield obstacle survey, although the holding points are surveyed and drawn into the airfield plan of course.
I don't think its really covered by the bit in CAP232 chapter 10 that says:
"Features to be surveyed:
Roads, railways, river or canal features shall have sufficient levels to show their
surface elevation, (in the case of a body of water subject to tides, high and low tidal
variations are required) and the height of the highest mobile feature that could be
expected on them. The features shall include vegetation, hard, mobile and
temporary objects"