How it can be done

When you right- click a file or folder for a nautilus script, the selected file is passed as an argument to the script. In most cases by something like:

import os

subject = os.getenv("NAUTILUS_SCRIPT_CURRENT_URI")

...using python3, in its simplest form.

If you replace this by:

import pyperclip



subprocess.call(["xdotool", "key", "Control_L+c"])

subject = pyperclip.paste()

...the currently selected file is used inside the script as an argument

What you need

To use this solution (16.04 and up), you need to install both xdotooland python3-pyperclip:

sudo apt-get install python3-pyperclip xdotool

The complete script, mentioned in comments

then becomes:

#!/usr/bin/env python3

import subprocess

import os

import sys

import pyperclip



# --- set the list of valid extensions below (lowercase)

# --- use quotes, *don't* include the dot!

ext = ["jpg", "jpeg", "png", "gif", "icns", "ico"]

# --- set the list of preferred filenames

# --- use quotes

specs = ["folder.png", "cover.png", "monkey.png"]

# ---



# retrieve the path of the targeted folder

subprocess.call(["xdotool", "key", "Control_L+c"])

dr = pyperclip.paste()



for root, dirs, files in os.walk(dr):

    for directory in dirs:

        folder = os.path.join(root, directory)

        fls = os.listdir(folder)

        try:

            first = [p for p in fls if p in specs]

            first = first[0] if first else min(

                p for p in fls if p.split(".")[-1].lower() in ext

                )

        except ValueError:

            pass

        else:

            subprocess.Popen([

                "gvfs-set-attribute", "-t", "string",

                os.path.abspath(folder), "metadata::custom-icon",

                "file://"+os.path.abspath(os.path.join(folder, first))

                ])

Adding this to a shortcut key will set the icons for all directories inside the selected one.

Adding it to a shortcut key (!)

Adding shortcut keys, running (scripts using-) xdotool commands to press another key combination can be tricky. To prevent both key combinations to interfere with each other, use:

/bin/bash -c "sleep 1 && python3 /path/to/script.py"

Explanation

When Ctrl+C is pressed while a file is selected, the path to the file is copied to the clipboard. We are simulating the key press with:

subprocess.call(["xdotool", "key", "Control_L+c"])

python's pyperclip module simply produces the path, stripped from file:// when using pyperclip.paste() (this will not literally paste, but make the path available inside the script).