How to manage remote music library but at the same time have some music still on the MacBook (preferably in sync)? How can I keep my whole music library on my NAS and comfortably access it and manage it there and at the same time have a music selection locally stored on my MacBook that I can manage while away and that gets synchronized with my NAS library when I get back.

Is there way to do that?

My Setup: a Synology DS412+ with DSM 4.1 and a MacBook Pro with Mountain Lion

Update:
The idea is to have a master library on the nas and a slave library for mobile use that merges all changes back to the master and gets updated information from the nas. Like a selective cloud sync.

Update (2): 
The purpose of this usage is to be able to play music without having access to broadband internet (train, plane, car, public places, ...) without maintaining two libraries.
You could set up something like this (Make sure that your iTunes Library File (iTunes Music Library.xml) is on your mac, not on your NAS):


Create a playlist "On the go"
Use this script to move the files in the playlist from your NAS (iTunes media location) to your mac. 
On the go, with no connection to your NAS: iTunes will tell you that the files are not available when you click to play, click "search" and select the folder you´ve synced the files to. 
When you add new music on the go, iTunes will copy that to the default location  (/music/iTunes/) on your mac.
When you have access to your NAS again: Check that the iTunes Media Location is your NAS (Settings > advanced) and click File > Media Library > Organise Media Library. All Files you added while on the go will be copied to your NAS. 
Delete the Files in your local iTunes Media Folder.