So I recently created a small tool to view your opponents BattleTag, it happens to be stored in the memory. Included in the .rar is the source code and a compiled .exe, the source is included so that a person may compile it themselves if they do not feel safe with the .exe I have made.
http://goo.gl/UrHl3d
Source:
http://goo.gl/SYNR2V
Edit: Updated for usernames with a small amount of characters not finding the correct data.