How do you enter a half comma (顿号, dùnhào) with Google Pinyin IME and a French keyboard
Since I figured it out after some more searching and trying, here is the answer:

After starting Google Pinyin IME, right-click on the keyboard button (软键盘 ruǎnjiànpán: software keyboard). From the menu that appears, choose 标点符号 (biāodiǎn fúhào: punctuation marks). This will bring up a keyboard on the screen; the dun comma is on the top row, on the same key as the number 2. You can than enter the dun comma using either the software keyboard or your hardware keyboard. You can then close the software keyboard again by clicking on the keyboard button. 

Unfortunately, I am not aware of keyboard shortcuts to do this.