BJ...I've tanned cased skins this same way too before but never put them on a stretcher to dry.I can see how it could work for you though.Without any hand work done to them I don't think just putting them on a stretcher softens them.Actually for me they are easier to soften opened up split.
Well I checked back when I started this hide from a raw or green state.Exactly a week and it's done.I soften them breaking and stretching wise as it dries every half hour or so.
Some clumsy skinning holes I fix with a baseball stitch while wet before stretching.You cannot see the stitch line from the hair side.
Well that's it.It is'nt a tan that gets as soft as my brain tan I don't think but it definitely will serve the purpose for a number projects.