OBJ format: can you export faces with indexes relative to the last "o Something" (object)? When I export a model with multiple objects, the face indexes of the second object, for example, won't start with 1, because they are "global", which means you have to keep track of every (v, vn, vt) occurrency. I want blender to export .obj and have faces indexes starting at 1 each time, even if it's necessary to duplicate some vertex. Is it possible?
PS.: I know I could export them separately, but that's not practical.
I do not understand the question. Can you specify why it is impractical for your specific case. Where and how do you plan to import the data?

Obj files will need a unique face index as different objects are separated by obj groups referring to the vertices which are included in each object. Otherwise the file would result in something different when importing somewhere else. If you want to alter the way the wavefront obj format is imported or handle the data externally in a specific way you could always parse the file for obj groups using python and sort it the way you would like for your specific application. Alternatively you could also export all objects separately using python, obviously depending on the constraints you have making it impractical for your specific case.