IMHO, the '51 convention is big endian - all of the '51 asms I tried did it this way (not that I tried all of them).
Of course there is little direct implication in the '51 core itself, as there are almost no 16-bit data instructions (only mov dptr,#xxxx and stack push on call, but both are quite irrelevant for the case); all of these is a convention only.

But, this convention is completely in line with Intel's complete confusion plan - 8080 is little endian, so why would an another 8-bitter of the same era be similar? :-)))

JW