Boustrophedon Order

If niching else, it seems like changing directions would make it pretty clear which way a given line is meant to be read (unless it contains a hugely unlikely proportion of mirror-symmetric characters like A, O, M, &c.)

I am guessing here: once we are proficient at reading we start recognising word shapes, not letters. In the mirrored writing the word shape as a whole does not change - it's just mirrored, which makes it easier to recognise than if the letters are reversed.

Here's the text from your link. Which is easier to read, this or the PNG?

'This example of boustrophedon text

eht rof yllacificeps nettirw saw

Wikipedia article on this ox turning

txet htiw llaw a gnirevoc fo dohtem

in ancient Greece and elsewhere.'

I recall that the Moai (https://secure.wikimedia.org/wikipedia/en/wiki/Moai) were transported horizontally quite a ways from their quarry to their pediments. >IF< the inscriptions are on these prone stones then perhaps it was so that two stone carvers could carve two lines simultaneously thereby halving the time to complete their work. By inverting the second line the second carver doesn't have to write upside down. The carvers just face each other while squatting on the stone's face and start at opposite sides of the face working past each other.

If this style of inscription occurs frequently on stone surfaces not likely to be transported then this theory is unlikely.

EDIT: answering the reply below - the above technique can be applied to any number of lines. As in pipeline processing for vector computers (https://secure.wikimedia.org/wikipedia/en/wiki/Pipeline_%28s...) the last operations do not fully utilize the available capacity of the system. In the case of inscribing in stone - the last line only needs the labor of one carver.

As for frustrating - I'd find carving in stone quite tedious in any case.

I may still be mistaken but I don't think it's for the reasons you gave.