Well, I thought too to use the Matrix laserdisc audio, but, as you wrote, it's terribly expensive so I'll leave its soundtrack out - unless someone who has it would like to capture it, preferably bit-perfect...

A french VHS Hi-Fi stereo track is not a bad idea; if only I have the italian VHS, I'll do it the same; but, as I have all the BD soundtracks, and it seems that audio was the same on DVD and BD, I could think the VHS should be the same, apart the fact that it is only stereo, Dolby Surround encoded, while the BD soundtracks are 5.1 discrete surround... so I think this time I'll use the untouched BD tracks.

Yes, upscaled DVD is one choice, the other is to use CM chroma over the BD luma (at correct levels).

Keep an eye on this thread for the next updates.