The original 35mm print we used only had Stereo audio - and that was in Spanish and we included it in this version - but had it been an English print it almost certainly have been in Stereo (mono would have been exciting). The whole idea of the project was to recreate the experience of watching the print on a 35mm projector, without you having to buy the print or the projector. Was there 5.1 surround in '77? I don’t think so, the whole reason there is a mono mix is because a lot of cinemas didn’t even have stereo yet.
So, to answer your question, there was no attempt to use the DTS-HD MA 5.1 track, however, I don’t see any technical reasons why it could not have been used. The Blu-ray structure is made up of tracks, each track uses a playlist and each playlist is made up of clips. When seamless branching, you must first make sure that all the audio and video streams that are in each clip are encoded using identical settings, preferably by the same software. You will have a hard time seamlessly branching an Mpeg2 clip with AC3 audio together with an h.264 clip with PCM audio. What this means in practical terms is that all the audio and subtitle streams must be split to match the video clips. It would be nice if we could add a single audio or subtitle track to a completed video playlist, but unfortunately audio and subtitles are assigned at the clip level. So, for example, all of the subtitle tracks had to be split at the end of the flyover and then re-timed for the rest of the film, and all of the audio and subtitles streams had to be placed in the same order on each clip, and assigned the same language codes, lest the audio or subtitle language suddenly change at the branch.
So to use the DTS-HD MA 5.1 audio, you would first have to find a way to split it between titles-crawl-Flyover and the rest of the film. You may be able to do this with ffmpeg, just literally splitting the file without any re-encode and that would certainly be what I would try first - you never know it might even work. But if that failed, then I would use EAC3to to extract all the channels to wav files. I did this once before when I made those Back to the Future audio tracks with the swearing removed so my kids could enjoy them too. What I discovered then was that, for whatever reason, EAC3to mixed up all the channels, or at least mislabeled them. But anyway, the next thing to do would be to bring the wav files into something like Adobe Audition, and put them all back together as a 5.1 mix, correcting the channel placements if needed. Then split the mix at the end of the flyover, and save each part as a new 5.1 PCM mix.
At that point, you could simply bring in the new wav files and use them as 5.1 PCM audio tracks, or you could re-encode each one as DTS-HD MA, and then bring them into the authoring software or perhaps your authoring software may even be able to do the DTS encoding for you. I wanted to use PCM audio, and had room to do so, so I didn’t look into that.