Here's how you can set up a push to talk button for G+ in Windows.
1) Go to
http://www.pushtotalk.nu/ and download the small software.
2) Extract to a folder.
3) Run the software as administrator.
4) Next to Current Hotkey, click and set the key you want to use.
5) Go to another window and test it!
6) You're done!
Optionally, you can also set up fancy sounds for when you trigger it on and off.