I don't know if either you still play at BGRoom or if you've figured out how to get the .mat file from the client, but this is how you do it:
1) My account
2) Game History
3) Click on the hyperlink for the match ID you want the file for (i.e. 23231400)
The board will show up as if you're playing but lets you cycle through the moves. At the top right, under the resignation x, is a button with an arrow and "GNU." Click this and you will be given the option to save the .mat.