I'll share a couple of thoughts on this, since I own the same pack. My Feather pack is a large and my torso measurement is around 21".
I use the "V" shaped belt strap (not the pocket belt option). The attachment points along the sides of the pack are 4 1/2" apart. If someone were to attach a webbing belt strap to the lower attachment only, it seems that it would lengthen the torso by 2 1/4". Somebody please correct me if I am wrong about this.
I would hesitate to put load lifters on this model, or to pinch the fabric together above the shoulder strap attachment point. It would interfere with the zip closure top option. Although, the pack could still be used with it's roll top option, if fabric was pinched together.
Good luck with the sale. Somebody is going to get a very cool pack!