I also tried this with \voiceOne etc (with version 2.8.8, admittedly),
and couldn't get the first chord to print as 4 different voices of
which three have upward stems. Lilypond gives me "warning: ignoring
too many clashing note columns", and collapses all three into one
chord. If we would have \voiceFive, that would solve the issue.
If you look at the definition of voiceXXXX you will see:
voiceOne = #(context-spec-music (make-voice-props-set 0) 'Voice)
voiceTwo = #(context-spec-music (make-voice-props-set 1) 'Voice)
voiceThree =#(context-spec-music (make-voice-props-set 2) 'Voice)
voiceFour = #(context-spec-music (make-voice-props-set 3) 'Voice)
So voiceFive shoudl be possible :-)
Bert