What Aaron suggested seemed to work with the passage similar to the previous rit-a tempo example, but the stretching 8-notes problem arises elsewhere, like in the example below.
\version "2.18.2" pocoaccel = \markup { \italic "poco accel." } common = { \override TextScript.extra-spacing-width = #'(0 . 1.5) \override TextScript.extra-spacing-height = #'(1 . 1) \override TextScript.staff-padding = #1 s1 ^\pocoaccel } oboe = \relative c''{ g8 (a b c d) g, (b c) a1 } \score { \new Staff <<\common \oboe>> }