Permission to base a film on Charlie Chan would be possible if a go-ahead were granted by the current holder of of the rights to the character, which is, at his time, an entity called Leisure Concepts, I believe. While the original Chan stories may, as you note, be in the public domain in other parts of the world (we have a link to an Australian source to the stories online here at this humble website), that not, perhaps, so with the film rights.