figuredBassCenterOnNote = \override BassFigure #'X-offset =
#(lambda (grob)
 (let* ((paper-col (ly:grob-parent grob X))
        (elts (ly:grob-object paper-col 'elements))
        (figure grob))

   (for-each
    (lambda (idx)
      (let ((elt (ly:grob-array-ref elts idx)))
            (set! figure elt)))
    (iota (ly:grob-array-length elts)))

   (-
     (interval-center (ly:grob-robust-relative-extent figure figure X))
     (interval-center (ly:stencil-extent(ly:text-interface::print grob) X)))))


\new Staff << { \clef F c4 c c c c2 c c1 c c c \bar "|."}
  \figuremode {
    \figuredBassCenterOnNote
    <5>4 <6 4> <6 5> <_+> <6 4>2 <5> <_+>1 <5> <6 4 2> <6>
  } >>