Quoting Paul Scott <address@hidden>:
At least in the last several versions of the manual (this is from
2.9.12) the following appears implying that you can't have both UP
and RIGHT or DOWN and LEFT:
Yes, that's correct. For some layout objects, the direction property
works horizontally (so you can get LEFT or RIGHT or any numerical
value inbetween or even further left or right) and for other objects
the direction property works vertically (so you can get UP or DOWN).
Note that, internally, both #LEFT and #DOWN are shorthands for the
numerical value -1.0 and similarly, #UP = #RIGHT = 1.0, so you if you
want to confuse the reader of your .ly file, you can for example use
\override BarNumber #'direction = #LEFT
which will result in bar numbers placed below the score lines.
Note also that for technical reasons we cannot have different
versions of the documentation texts of the direction property
for different objects, even though it would be more pedagogical
to only mention #UP and #DOWN for objects where that's relevant
and #LEFT and #RIGHT for the rest.