Yes the concept of "Endemism" can only have meaning in relation to some defined area. In relation to birds I had used this concept with reference to countries, Islands/Groups of Islands and some regions.
Apparantly such "loose use" doesn't appeal to the birding pedants and they, under the banner of "Birding International", have developed the concept of "Endemic Bird Areas" (EBAs). There are 218 of these around the world where, a maximum area of 50k sq kms "encompasses the overlapping breeding ranges of restricted-range bird species, such that the complete ranges of two or more restricted-range species are entirely included within the boundary". There is a secondary list of areas containing at least one restricted-range endemic bird species, but not qualifying for the full Endemic Bird Area status.
As you can imagine the world's birders are as keen on questioning these areas as we are in questioning WHS connections and the areas are subject to flux in the light of new information on breeding and sightings and on speciation decisions!
Wiki has the following links
a. Primary Areashttp://en.wikipedia.org/wiki/List_of_Endemic_Bird_Areas_of_the_World
b. Secondary Areas http://en.wikipedia.org/wiki/List_of_Secondary_Areas
As far as I can see there are no areas in Thailand let alone covering Khao Yai.
Our list to date hasn't tried to follow this list and relates almost entirely to small(ish) islands (some of which are on the list and some not) plus Cuba and Sri Lanka - both of which ARE on the list. As Khao Yai doesn't alter the situation I would leave things as now - but the problem will return when someone asks about e.g. the endemics of Lope and Dja either/both of which might be in the EBA No 85 of "Gabon and Cameroons Lowlands"! If they can list the birds involved then good luck to them! The only worry might be that a real birding enthusiast could make the Connection very long if a lot of the 218 areas are represented by parks on the List - but so what if it is correct? It would show which WHS contained "endemic birds" (and which birds) as defined by Birding International and that can be of interest/note. But I don't think we want to extend the concept to endemism in butterflies, snakes, trees, fish, insects, .....! That may be "speciesism" but a line has to be drawn!