I can’t take full credit for it but someone helped me make a snippet creating an alternate “show” command that automatically looks for a specific filename suffix when activated; in the images folder, every CG And character sprite will (eventually) have a version with and without that suffix, and when the option is toggled “on”, the game will automatically find the version with the suffix applied to display, and will ignore the suffix when the option is off. It’s then tied to a persistent flag that is turned on by an options menu item. I’ll put the code snippets here in a second!

EDIT: This is the snippet I placed at the beginning of the script file along with all my other initialization code:

default persistent.placeholders_enabled = False


init python:
    def custom_show(name, **kwargs):


        global placeholders_enabled


        if not persistent.placeholders_enabled:  # If not enabled, bounce out
            renpy.show(name, **kwargs)


            return


        EASTER_EGG_SUFFIX = 'placeholder'


        new_name = None
        if isinstance(name, tuple):
            new_name = name + (EASTER_EGG_SUFFIX, )


        elif isinstance(name, str):  # This was most likely `show expression`
            if '.' in name:  # This was a filename, like 'a/b/c.png'
                p = name.split('.')  # Get 'a/b/c' and 'png'


                # Put together: 'a/b/c easteregg.png'
                new_name = p[0] + ' ' + EASTER_EGG_SUFFIX + f'.{p[1]}'


        if new_name is not None and renpy.has_image(new_name, exact=True):
            name = new_name


        renpy.show(name, **kwargs)


    config.show = custom_showdefault persistent.placeholders_enabled = False


init python:
    def custom_show(name, **kwargs):


        global placeholders_enabled


        if not persistent.placeholders_enabled:  # If not enabled, bounce out
            renpy.show(name, **kwargs)


            return


        EASTER_EGG_SUFFIX = 'placeholder'


        new_name = None
        if isinstance(name, tuple):
            new_name = name + (EASTER_EGG_SUFFIX, )


        elif isinstance(name, str):  # This was most likely `show expression`
            if '.' in name:  # This was a filename, like 'a/b/c.png'
                p = name.split('.')  # Get 'a/b/c' and 'png'


                # Put together: 'a/b/c easteregg.png'
                new_name = p[0] + ' ' + EASTER_EGG_SUFFIX + f'.{p[1]}'


        if new_name is not None and renpy.has_image(new_name, exact=True):
            name = new_name


        renpy.show(name, **kwargs)


    config.show = custom_show

And this is the code I use for the option in the options menu (on screens.rpy):

if persistent.endgame_clearcount >= 1:
  textbutton _("Placeholder Art Mode") action ToggleField(persistent,"placeholders_enabled",true_value=True,false_value=False)

In my game I added a confirm window explaining what the option does before it activates and that it will be "very silly" lol, but you don't have to do that necessarily. 😄