For stamping I would think you could use the Higgins permanent colored inks like are sold for art projects. They are not very expensive and come in a number of different colors. DS has used them for calligraphy and mopped up spills did not wash out and were not stiff. Lol!
You could also try the Febings (sp?) leather dyes. I know they are permanent and do not make the fabric stiff. Both DH and one DS are leather crafters and they use old T-shirts to apply the dyes.