1. Take the '93 mix as your base.
2. Remove all the '93 additions (listed here) by patching in the '77 Dolby Stereo mix.
3. Run the patched file through a software pro-logic decoder to obtain left, centre, right and surround channels.
4. Examine the boom track from the '97 SE mix to find any relevant low frequency effects; edit the boom track so it syncs with the base track.
5. Examine the rear channels from the '97 SE mix to find any surround effects that match up with the surround channel from step 3. Since the effects from the 5.1 mix are likely to be full frequency, they can be used to replace the frequency-limited audio extracted from the 2-channel matrix-encoded file.
6. Maybe you will need to apply some equalisation to the tracks to get a consistent balance?
7. Encode all tracks as a Dolby Digital 4.1 stream.
Like I said, this is just me theorising, but it's intersting to speculate if something like this could be done. (Also note that by all accounts, the 2004 5.1 mix is best avoided as a source).