Currently, Backdrop makes use of the functions
field_defaults_instance() to provide default setting arrays for field and instance. These two functions (not present in Drupal) are hardcoded and the arrays they return cannot be altered in any way, making it impossible to provide custom settings for field info or instance, change the default value for a setting, or add a new settings "key" for a custom settings. It would be pretty straightforward to add an alter hook call before returning these arrays (and ideally we would need to pass additional contextual information to these functions and to the the alter calls). I'm wondering what your thoughts are about doing something like this, and/or if it's contemplated anywhere.