There isn’t any content to make a 6.1 mix, since the original material has only a mono surround channel. In theaters this would be played by the entire array of surround speakers, producing a very wide sound filling the entire side and back areas of the room.
In order to approximate this in a home environment, with a typical 5.1 setup, I used the output from Prologic II in movie mode and then manually inverted the polarity of the right surround channel. Doing this prevents the mono surround effects common to both rear channels from forming a phantom center behind the listener, making them wider and more diffuse, as was the intention. Mono surround films typically aim to create a more ambient and less easily localized effect than modern formats, which can pan surround effects in any number of specific directions. (Inverting the right surround also has the side effect of making the upmix at least somewhat listenable if it is downmixed back to stereo again.)
The number of channels used in the delivery format is less important than its ability to reproduce the aesthetic used in initially creating the mix. Sending the mono surround effects to speakers located only in the back of the room would defeat the intention and would have little similarity to the way it was presented in theaters.
If I end up making another version at some point, I may use some different techniques than the existing version, in order to reduce front channel crosstalk into the surrounds and make the LFE more authentic, but the overall approach would still be similar. It is possible that a 4.1 mix might actually be more friendly to those with 7.1 systems, since it is possible to expand it during playback: according to Roger Dressler, playing a mono surround film with the receiver set to Prologic II in music mode will spread the surround effects equally among all four rear speakers. Note that this must be done in music mode, NOT movie mode, or else they will be sent only to the back, which is not at all what we want. This trick will not work with my current version, since the surround effects are decorrelated from each other, and I don’t know how that would sound since I only have a 5.1 system myself.