I finally found an answer on Wikipedia:

<quote>
Little endian processors order bytes in memory with the least significant byte of a multi-byte value in the lowest-numbered memory location. Big endian architectures instead order them with the most significant byte at the lowest-numbered address.
</quote>

IOW, both endians will start at the lowest-numbered memory location with (for this particular type) the unused bytes being at indexes 10 and upwards.
This is the best outcome (because it makes things simplest) and also is consistent with what I've done in Math::MPFR.

Cheers,
Rob