NSURL category that provides a simple way to append a fileURL to the end of favorite items or favorite volumes list used in the side bars seen in Finder and the open and save dialog boxes.

NSURL+FavoritesList.h

//
//  NSURL+FavoritesList.h
//  FavoritesListTester
//
//  Appends the URL to the end of the Favorite Items or Favorite Volumes list
//  (found in the side bar of Finder windows and open/save dialogs, etc.)
//
//	Notes:
//
//  1. Must be a file URL.
//  2. Appends only (no insert at top for simplicity).
//  3. Compatible back to Mac OS X 10.5.
//  4. Due to Finder bugs over the years, this category doesn't bother
//     allowing for setting custom icons/names; it just uses the default Finder
//     icon and file name for simplicity.
//
//  Created by Joshua Nozzi on 6/23/15.
//  http://joshua.nozzi.name/2015/06/23/nsurl-category/
//  https://gist.github.com/jnozzi/817f3276c55dbb7025be
//
//  Copyright (c) 2015 Joshua Nozzi. All rights reserved.
//  Released under BSD license (this notice must be retained if redistributed).
//  No guarantees made. Use at your own risk.
//

#import <Foundation/Foundation.h>

@interface NSURL (FavoritesList)

- (BOOL)appendToFavoriteItemsList;
- (BOOL)appendToFavoriteVolumesList;

@end

NSURL+FavoritesList.m

//
//  NSURL+FavoritesList.m
//
//  Appends the URL to the end of the Favorite Items or Favorite Volumes list
//  (found in the side bar of Finder windows and open/save dialogs, etc.)
//
//	Notes:
//
//  1. Must be a file URL.
//  2. Appends only (no insert at top for simplicity).
//  3. Compatible back to Mac OS X 10.5.
//  4. Due to Finder bugs over the years, this category doesn't bother
//     allowing for setting custom icons/names; it just uses the default Finder
//     icon and file name for simplicity.
//
//  Created by Joshua Nozzi on 6/23/15.
//  http://joshua.nozzi.name/2015/06/23/nsurl-category/
//  https://gist.github.com/jnozzi/817f3276c55dbb7025be
//
//  Copyright (c) 2015 Joshua Nozzi. All rights reserved.
//  Released under BSD license (this notice must be retained if redistributed).
//  No guarantees made. Use at your own risk.
//

#import "NSURL+FavoritesList.h"

@implementation NSURL (FavoritesList)

- (BOOL)appendToFavoriteItemsList {
  return [self appendToList:kLSSharedFileListFavoriteItems];
}

- (BOOL)appendToFavoriteVolumesList {
  return [self appendToList:kLSSharedFileListFavoriteVolumes];
}

- (BOOL)appendToList:(CFStringRef)list {
  
  // Pessimism ...
  BOOL result = NO;
  
  // Do we have a file URL?
  if (self.isFileURL) {
    
    // Ask CoreServices for the favorite items list
    LSSharedFileListRef listRef = LSSharedFileListCreate(NULL, list, NULL);
    if (listRef) {
      
      // We've got the list, so try to append our item
      LSSharedFileListItemRef itemRef = LSSharedFileListInsertItemURL(listRef, 
                                                                      kLSSharedFileListItemLast, 
                                                                      NULL, 
                                                                      NULL, 
                                                                      (__bridge CFURLRef)self, 
                                                                      NULL, 
                                                                      NULL);
      
      // Did it work?
      if (itemRef) {
        
        // Release the item and flag success
        CFRelease(itemRef);
        result = YES;
        
      }
      
      // Release the list
      CFRelease(listRef);
      
    }
    	
  }
  
  return result;
}

@end

LSSharedFileListCreate, etc. appear to be deprecated starting with OS X 10.11. Any idea what the replacement is?

It looks to me like the idea is to replace this type of access with "Finder Sync".
https://developer.apple.com/library/mac/documentation/General/Conceptual/ExtensibilityPG/Finder.html

Although I haven't found in there yet how to append to the favourites sidebar.