fwiw. i just had my memory jogged that Disc 2, the Theatrical Cut, from the 3 Disc Special Edition, does indeed have LPCM audio. Disc 1, the HD remastered version (w/ 2012 mixed in) has only DD.
not sure why they gave the remastered version DD except that the video encode probably didn't leave enough of the 10 Mb bitrate max allowed on a DVD to also have PCM, so the audio needed compressed. I haven't really tested it, but this seems like a likely reason. (I have had R2J DVDs that wouldn't mux with just adding subs due to bitrate spikes that were too high, so i had to use lower quality subs or transcode audio just to subtitle the disc -- some of these retail discs are already over spec in the retail version).
So disc 2 can be used for lossless audio for removed BGM (tho lower fidelity and sampling rate, so it needs resampled), but I still have no idea what the hell to do for a 5.1 mix other than skip it. (a mushfake 5.1 upmix with plogue bidule likely wouldn't cut it to match sonically at all).
regardless, if someone else undertakes this, I wish them good luck and I hope to find a copy of their work to watch.