I would find such a feature useful and was writing about its usefulness already in at least one thread here. The difficulty is to find an implementation that fits into the darktable philosophy. One aspect I did not find a solution for is that it should be a feature that is not solely reflected in the database but could be restored from the xmp files. An idea would be to give every picture a unique ID and when resorted every picture could hold entries of the next element in the list. That way, it could be represented in the xmp, probably some error recovery strategies would be required if the xmp state is not reflecting something useful and what should happen if an image is physically moved and no longer available (e.g. moved to an external harddisk). However, that whole feature would only be really useful if a “collection” feature is implemented first, otherwise it would just link several images together and be nothing else as an ordered grouping feature. The collection would specify the context in which a special custom sorting would be used. Therefore, one picture could hold several custom sorting sets that relate to different collections. The special sorting should then be added to the standard sorting options whenever a collection is selected. Something like that:
Picture: a
UID: 0815
Collection: "photobook" next 4711
Collection: "presentation" next 4712
Picture: b
UID: 4711
Collection: "photobook" next 4712
...
Hope this makes sense. However, I am not a dev and therefore not capable of implementing something like that, but I think it should be possible based on tags if a namespace for it is defined in the tags (probably a subfolder of darktables own tag namespace). If images have to be moved around with the mouse, the UI implementation would probably require to be solved in darktable directly and not e.g. with a lua script. Same for the sorting itself.
The more I think about, the more I think these are even two independent features, the sorted groups and the sorted collections, probably there’s a need for both of them …