Well, for those interested, the answer seems to be YES.
I have just installed DSM 4.0, and can now stream my FLAC files from Audio Station to my NAD C446. Using the DS Audio app for Ipad/Iphone makes it for a very nice interface, much better than PlugPlayer (It does not stop playing when you Iphone/Ipad goes to sleep !)
Now, the only question is whether this is true bit for bit streaming; hopefully the file is not converted on the way (I have not actived transcoding). On its screen, my NAD shows the file playing as a .flac one, so would indicate the NAD actually receives the original FLAC file.
I hope this helps others, give it a try; really much better than using Media Server and then controlling with PlugPlayer.