the troubles you encountered with invisible models should've shown up in the log file, You can alt+tab out of the game and open the log file when the game is running. Then you'll get very good clues as to what is causing the problem. But since you found another way to do it, I mention this for future references
I think you've run into a engine limitation issue since the uniform only changes the body, not the head. To do that you would need to create the balaclava as a helmet that fits in the helmet slot. The drawback is that if you wear it you wont be able to wear a helmet.
Since I never actually did it myself, I've only talked about it with my Russian friends I'm entering unkown territory so what I say may not be exactly correct. Anyways, here's what I think you should do.
You're on the right track by adding it to the AZP pack, and you need to add it to all the RPC AZP packs(36?). What you also need to do is add it to the EQUIPMENT.INI file under the HeadArmour entry, that should make the model swapping happen.
Sorry, but that's my limited knowledge on the subject, I hope it helps.