Sure. You need a DLNA server running on the NAS and the BR player has to be DLNA capable. Many NAS manufacturers provide their own DLNA server; if you don't have one many people start with Twonky.
Mkv is not part of the DLNA standard; however, some manufacturers add it in. So if both the NAS and the BR devices added it, then yes. You might be better of getting something like the WD TV Live and not bothering with streaming. It's easier to set up and will do what you want; the only down side is it's another device to connect, and cost money. http://www.amazon.com/Streaming-Media-Player-Wi-Fi-1080p/dp/B005KOZNBW/ref=sr_1_1?ie=UTF8&qid=1359195628&sr=8-1&keywords=wd+tv+live