You could try the bell on the door thing. I have seen lots of dogs train that way. Whenever you take them out to potty, ring the bell (jingle bells) and when you come in hang them on the door. Once they associate the two, they will nose the bells to go out. There is a ton of info online with many different methods to teach you how to do this. Then you don't have to worry about them barking. It took my Corso forever to figure out his bark. He is also silent about asking to potty. Dogs by nature are not a vocal species and work better with body language (that's why it is easier to train a dog with hand signals than verbal cues, they naturally look for body language). He just sits by the door. Every once and a while we get a sigh, but that's it.
If you want to teach them to bark in general because they aren't, don't panic, they'll figure it out. When I first started protection training, I could not get Ickis to bark during play (which is essential for the bark and hold!). He would bark when he saw people or dogs and would bark if someone came to the door, but not during training. He never barked when he played with out other dog, so teaching him to bark during play was difficult. He's got it now! One of us would hold him back by his lead (on a harness) and the other would 'tease' him with a toy on a rope (flirt pole). Well, he loves toys and especially loves playing. When the toy would move around he would go for it, but couldn't reach it. He would bounce around, try to grab it and miss. Day after day we would do this, only rewarding him with the toy for the desired behavior. Eventually, he got frustrated and barked, when he did, we all went nuts and he got a good round of tug-o-war and then the toy to run around with. He learned very VERY quickly, if he barks he gets the toy. Now, when we get out the tug toys or bite sleeve, he knows it's bark time!
That is just precious.