Example extensions for my 'Downloads for logged in users' plugin (https://wordpress.org/plugins/downloads-for-logged-in-users/)

allow-one-download-always.php

<?php
/*
Plugin Name: Allow specific download always
Plugin URI: https://www.damiencarbery.com/downloads-for-logged-in-users/
Description: With the <a href="https://wordpress.org/plugins/downloads-for-logged-in-users/">Downloads for logged in users</a> plugin, always allow one download.
Author: Damien Carbery
Version: 0.1
Requires Plugins: downloads-for-logged-in-users
*/


add_filter( 'spdownload_check_perms', 'dcwd_allow_specific_download_always', 10, 2 );
function dcwd_allow_specific_download_always( $user_logged_in, $post_id ) {
	if ( 2919 == $post_id ) {
		return true;
	}

	return $user_logged_in;
}

allow-wednesday-downloads.php

<?php
/*
Plugin Name: Allow unrestricted downloads on Wednesdays
Plugin URI: https://www.damiencarbery.com/downloads-for-logged-in-users/
Description: With the <a href="https://wordpress.org/plugins/downloads-for-logged-in-users/">Downloads for logged in users</a> plugin, allow unrestricted downloads on Wednesdays.
Author: Damien Carbery
Version: 0.1
Requires Plugins: downloads-for-logged-in-users
*/


add_filter( 'spdownload_check_perms', 'dcwd_allow_downloads_on_wednesdays', 10, 2 );
function dcwd_allow_downloads_on_wednesdays( $user_logged_in, $post_id ) {
	$day_of_week = date( 'w' );

	// Wednesday is 3 (Sunday is 0).
	if ( 3 == $day_of_week ) {
		return true;
	}

	return $user_logged_in;
}

downloads-counter.php

<?php
/*
Plugin Name: Downloads counter
Plugin URI: https://www.damiencarbery.com/downloads-for-logged-in-users/
Description: With the <a href="https://wordpress.org/plugins/downloads-for-logged-in-users/">Downloads for logged in users</a> plugin, count each download and display the number in a new column in the Downloads admin page.
Author: Damien Carbery
Version: 0.1
Requires Plugins: downloads-for-logged-in-users
*/


class DLIU_Counter {
	private $cpt_name;
	private $meta_key;


	// Returns an instance of this class. 
	public static function get_instance() {
		if ( null == self::$instance ) {
			self::$instance = new self;
		} 
		return self::$instance;
	}


	// Initialize the plugin variables.
	public function __construct() {
		$this->cpt_name = 'dcwd_simple_download';
		$this->meta_key = 'dl_count';

		$this->init();
	}

	// Set up WordPress specfic actions.
	public function init() {
		add_action( 'spdownload_after_download', array( $this, 'increment_downloads_counter' ) );

		add_filter( 'manage_' . $this->cpt_name . '_posts_columns', array( $this, 'add_count_column' ), 15 );
		add_action( 'manage_' . $this->cpt_name . '_posts_custom_column', array( $this, 'display_count_column' ), 10, 2 );
	}


	function increment_downloads_counter( $post_id ) {
		$download_count = get_post_meta( $post_id, $this->meta_key, true );

		if ( is_numeric( $download_count ) ) {
			$download_count++;
		}
		else {
			$download_count = 1;
		}

		update_post_meta( $post_id, $this->meta_key, $download_count );
	}


	// Add custom column to dcwd_simple_download post type admin list
	function add_count_column( $columns ) {
		// Insert the File column after the title
		$new_columns = array();
		foreach ( $columns as $key => $value ) {
			$new_columns[$key] = $value;
			if ( $key === 'protected_file' ) {
				$new_columns[ $this->meta_key ] = 'Download count';
			}
		}
		return $new_columns;
	}


	// Display the download count.
	function display_count_column( $column, $post_id ) {
		if ( $column === $this->meta_key ) {
			$download_count = get_post_meta( $post_id, $this->meta_key, true );

			if ( is_numeric( $download_count ) ) {
				echo wp_sprintf( '%d', $download_count );
				return;
			}
			else {
				echo '—';
			}
		}
	}
}
$DLIU_Counter = new DLIU_Counter();