I compiled it against current master. If your version of LilyPond doesn't have an X-positions property, you can assemble something that works like X-positions by getting the two spanner bonuds via ly:spanner-bound and then getting their relative coordinates.
\version "2.14.0"
\relative c'' {
\override TupletBracket #'stencil = #ly:slur::print
\override TupletBracket #'direction = #UP
\override TupletNumber #'whiteout = ##t
\override TupletNumber #'layer = #2
\override TupletBracket #'layer = #1
\override TupletBracket #'control-points =
#(lambda (grob)
(let* ((b-l (ly:spanner-bound grob LEFT))
(b-r (ly:spanner-bound grob RIGHT))
(crf (ly:grob-common-refpoint b-l b-r X))
(x-pos (cons (ly:grob-relative-coordinate b-l crf X) (ly:grob-relative-coordinate b-r crf X)))
(pos (ly:grob-property grob 'positions)))
(list (cons (car x-pos) (car pos))
(cons (+ (car x-pos) (/ (interval-length x-pos) 3)) (+ (car pos) 0.5))
(cons (+ (car x-pos) (* (interval-length x-pos) 2/3)) (+ (cdr pos) 0.5))
(cons (cdr x-pos) (cdr pos)))))
\repeat unfold 10 \times 2/3 { a4 b4 c }
}
In 2.14.0 the TupletNumber Y-offset callback doesn't exist, so I've nixed it. This messes with the tuplet number positioning, but you can write a positioning fixed based on the coordinates of the bracket. To get the bracket, if the TupletNumber is `grob' in a function, use (ly:grob-object grob 'bracket).