Found it! (Thanks to this thread, which had me looking at the original Z-specs again recently.) In Infocom’s Z3 spec, page 16, bit 0 of Flags 1 (which they call “the mode byte”) is the “byte-swap” bit: “0 means words are high-order byte first, 1 means words are low-order byte first”. All surviving story files have this bit set to 0, meaning big-endian.
In the modern spec, bit 0 of Flags 1 is unused in Z3. And in Infocom’s Z4 and Z5 spec, page 39, this bit is redefined as “upper window available” in Z4 and “color available” in Z5. (The modern spec defines it as “color available” for both Z4 and Z5.)