there is a paint called Createx that is used by all the airbrush artists to paint on leather and fabric. it is flexible but needs to be heat set. It comes in jars so it could be painted by hand..this is the paint of choice for leather jackets artists.
these are the instructions for use
Leather
1) Clean leather by wiping with mineral spirits or rubbing alcohol.
2) For best results, lightly spray mist a base coat of Opaque White on areas to be sprayed.
3) Apply color, avoiding over saturation and excessive paint layering.
4) Allow colors to dry thoroughly before heat setting.
5) Iron for 30 seconds at 225°using a protective cloth, or use a hot air gun at highest level for five minutes.