Actually, http://www.imperial-library.info/content/morrowind-preparation-corpse-volume-one-acquisition-corpse mentions how to acquire bodies in each land and a bit of the burial practices.
Hmm... Something stood out to me, there...
While few Bosmer perform Arkay's rituals when burying the dead, the more primitive Bosmer still practice cannibalism upon their enemies, which reduces the number of available corpses.
I guess the Green Pact isn't so inviolate anymore, after all.
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Anyway, to answer the OP's question, all forms of undead are soul trappable in all the games.
Of course, this may not mean as much as you might think, as all forms of undead are soul trappable, and so are Falmer just because they have a hunch in their back, or any other "creature", because Arkay's Blessing seems to stem from any character that is capable of wearing clothes, not from being of a specific set of races. Golden saints were "creatures" in Morrowind, then "NPCs" in Oblivion, so they were soul trappable in Morrowind, but not soul trappable in Oblivion. No explanation was given for the change.
In fact, dunmer can soul trap their own ancestor ghost in previous games... Repeatedly. Stick that in your "ancestor worship" and smoke it.