I've never seen a holder (sample: Voigtlander, Contessa-Nettel, KW, and Ica/Zeiss-Ikon Ideal types) that would allow loading the film into the sheath with the sheath in the holders. The KW holders are the closest; in the light, I might be able to work a sheet into the film sheath while it was in the holder, though I very much doubt I could do it in the dark. The problem is that the end of the film (barely) runs under the edge of the lip that retains the sheath on one end or the other, while needing to go under the fold at the edge of the sheath -- which requires the film to be flat and curved at the same time, a geometric improbability of high order...