Yes, I've done it, not with Black Ops II though, but with Modern Warfare.
There are some games that do not work with Dual Shock 2s through adaptor, like Gran Turismo and Metal Gear since they take advantage of the pressure sensor on the controller (like their previous PS2 versions but not compatible). None of the Call of Duty games use that feature so its safe to say it will work on Black Ops as it works on Modern Warfare.
The adapter we used (for two controllers) automatically assumes 2 slots when you connect it (not always the next available two), and the main problem is you don't know which is which because you do not have the "PS" button on Dual Shock 2s; so what we do is press the PS button on Dual Shock 3 and change it around until the Dual Shock 2s are between 1 and 4 (you test it after each change pressing "X" in the "Join" screen using the Dual Shock 2). This because the game (if it's anything like Modern Warfare and Tekken) will only recognize the first four controllers.