Alright, I've been looking for some free options for reducing the center channel of the AC3 file some more, and came across KnOckOut, a plugin for Audacity. Placing the center channel above the right channel, I then pulled the center channel to read as a left channel, and pulled the right channel to read as the right channel. This is so Knockout thinks that it is a simple stereo file. Playing with the settings allowed for a really impressive result. Leia's voice is hardly garbled at all, and except for a single errant note, I was hard pressed to hear any music. This is more than sufficient for incorporating into a trailer, methinks.
The only problem was that this didn't work. It only worked when I clicked the "preview" option, which played the first five seconds perfectly. Trying to apply the effect to the whole track did absolutely nothing except amplify the track. So I did what any sane person would do and recorded the five second previews with Windows Audio Recorder and stitched them together in Audacity. Here are the settings, if anyone else is interested in working from the AC3 audio:
Knockout settings for Leia Reconstituted Audio:
Vocals plus music on top, left panned. Right music channel on bottom, right panned.
Select both tracks, start Knockout. Extract Centre off, R input gain 13 dB, Output gain 8 dB, all other settings off.
preview section, record with windows audio recorder.
The moral of this story: It is quite possible to isolate the Leia hologram speech, with some effort.