Would you agree that the 'unavailable portion' as you've described it, is part of the Manuevering Area? If it isn't, why not? If it is, why would this area not be held to he standards of the rest of the manuevering area, such as surface integrity, clearance to position, etc?
Depending on the particular country's level of compliance with Annex 14 (including any differences they might have filed), it is possible that there could be Stopway prior to the (displaced) THR and perhaps RESA prior to the Stopway.
I have seen nothing in Annex 14 that specifies what the strength of these areas should be. As a result, in my country of operations, the Stopway is only half the strength of the RWY and, if there's a RESA, it is compacted gravel and soil, grassed over. I don't want to get into RESA too much, despite the fact that the Annex 14 specification states that it is provided for aircraft undershooting on approach, or an over-run on take-off. Many of the poorer countries do not provide RESA and this is why I don't want to discuss it in any depth.
However, the stopway is simply there to allow an additional length for an aeroplane to stop in the event of a rejected take-off, without doing any damage to the aircraft. Therefore, it is not intended to suffer the impact load associated with a landing and, hence, doesn't have to be as strong as the RWY itself.
The reality is that any airport operator that wants to save some money will use a lesser strength in the Stopway.
Now, to make this relevant to the permanently displaced THR scenario, the airport operator is not required to maintain the area prior to the THR to the same structual integrity as the RWY. Therefore, if the operator wants to save a quid, he won't bother to spend money on it as it deteriorates - short of patching potholes, etc.
I think this might suggest that you will never know the actual strength of the area before the displaced THR.