I made some leather using alder bark. not sure its what you want, if your after soft. You may want to try a small piece before you do that many hides. it seemed to me that the leather was pretty stiff done in the bark.
I peeled some bark off a tree, boiled it up and strained it let it cool and soaked the rawhide in it in a jar, can't remember how long, not very week maybe, but i did small strips, boot laces. good luck and looking forward to seeing how it turns out for you. Ed