Oops, to the whole list now...
In response to my own question, one can just set the positions of the
main beam as well, so actually, I think this works rather well. It's
certainly cleaner than Finale... Thanks Trevor for putting me on it
again!
\version "2.12.1"
\new Staff {<< { \oneVoice \override Beam #'positions = #'(1 . 3.5)
c'8*1/2[ d' e' f' g' a' b' c'' d''] }
\new Voice { \oneVoice \override Beam #'positions = #'(1 . 1.2)
c'8*1/2[ d' e' f' g'] }
\new Voice { s4 \override Beam #'positions = #'(1.2 . 3.5) \stemUp
g'8*1/2[ a' b' c'' d''] }
}