You can also just patch file "c:\Program Files (x86)\Battle.net\Battle.net.4124\battle.net.exe" with hex editor using following info:
Code:
file offset: old byte > new byte
000F9772: 8A > B1
000F9773: 4D > 01
000F9774: 0B > 90
There's also a possibility to patch wow.exe (offsets for x86, build 17688 below) to be able to reply to whispers sent from battle.net app:
Code:
004F1214: 74 > 90
004F1215: 2A > 90
004F121D: 75 > 90
004F121E: 21 > 90
I don't suggest anyone doing so though...
You can reply to whispers sent from battle.net app while playing D3 or SC2 without patching those game clients.