Hi Knute!
Don't know if this solution would be called clean and doesn't run in other problems, but using \voiceOne and \voiceTwo inside the variables to change voice-Styles seems to work for me. Had a similar problem a couple of months ago.
These were the changes I made:
rightHandUpper = \relative {
\voiceOne
gf'4._~ gf4 f16 ef ~ |
\voiceTwo
ef4.
}
rightHandLower = \relative {
\voiceOne
s2.
bf'4.
}
rightHand = <<
\global
\new Voice { \rightHandUpper }
\new Voice { \rightHandLower }
>>
All the best,
Christian