> isn't x64 as bi-endian as PPC? After all, it has big-endian load-store instructions.

Regarding PPC, there are different levels of little-endian capability: from none at all (970) over partial (60x, 7xx, 74xx, e600) to full (e200, e300, e500, e5500, e6500*, 4xx, PA6T, POWER7/8/9). See here (especially the linked PDF) and there.
As far as I can see, it's about data endianness vs. instruction endianness, where x64 implements only the former in the big variant. Whether big-endian data access is sufficient for the transparent emulation required in the case of MorphOS, I don't know.

* AltiVec in big-endian mode only