This can already implemented via  plugin using

EventListener.on_pre_save()

There's no reason for this specific functionality to be a part of the core API.

Would look similar to this (haven't tested):

import sublime_plugin

import re

class AutoUpdaterThing(sublime_plugin.EventListener):

on_pre_save(self, view):

r = self.view.find('regex_pattern_here', 0)

self.view.replace(edit, r, 'whatever you want dynamically inserted'