\score {
  \context Staff \notes \relative c' {
     \time 3/4
     % Here's what I was trying to do:
     < {\stemDown <g4 b g'> <f a f'> <e g e'>}
       \context Voice=vii {\stemUp d'2.} >

     % Or:

     < {\stemUp <g4 b g'> <f a f'> <e g e'>}
       \context Voice=vii {\stemDown d'2.} >

     % And here's what worked:
     < {\stemDown <g4 b g'> <f a f'> <e g e'>}
       \context Voice=vii
       {
         \property Voice.NoteColumn \override #'force-hshift = #1
         \stemUp d'2.
	 \property Voice.NoteColumn \revert #'force-hshift
       } >
  }
}