How to manage remote music library but at the same time have some music still on the MacBook (preferably in sync)?
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.