/modules/script_callbacks.py defines:

def on_infotext_pasted(callback):
    """register a function to be called before applying an infotext.
    The callback is called with two arguments:
       - infotext: str - raw infotext.
       - result: Dict[str, any] - parsed infotext parameters.
    """

It tells nothing about how do I read back my extra parameters (that I have set by assigning to p.extra_generation_params['…'] during image creation) and set my fields back to correct values, when the infotext is parsed from prompt field by dedicated UI button (or by "Send to txt2img" button, for example).

The callback is not bound to my class X(scripts.Script):, so I cannot use created components in def ui(self, is_img2img):, nor I can directly ask Gradio to change my components, can I?

Surely I can change the prompt by assigning to result['Prompt'], but that works only for default predefined fields. Am I missing something?
I've read generation_parameters_copypaste.py, but still couldn't understand how to restore custom fields from infotext.