I skimmed through this thread so I apologize if I am redundant at all.
I think that teaching a solid "leave it" and "watch me" would be very helpful for you. I find that clicker training works best for these commands. If you can get these commands down, when you spot another dog tell him "watch me". Keep his focus on you and praise and reward him as you walk by for paying attention to you, and not the other dog. It is a LOT easier to get a dog's attention on you and reward good behavior before they are in that state than it is to break him of that state and stop it from escalating into a lunge/bark, etc.
I have a friend with a DR dog and she has benefited from long walks with me and my dog. We walk them on opposite sides of the street for a while. When she fixates, she gets a watch me command. Once her attention is off of my dog, she gets praised and rewarded. We continue until she is comfortable with us walking beside her, and can look at my dog calmly. without reacting. Then we slowly come closer and closer. Eventually, we can walk side by side perfectly fine, and they even play a little bit (on leash). It starts to take less time for her to become comfortable with his presence. I think that this is good for her because it shows her the acceptable way to interact with another dog. On a walk alone, she doesn't get any interaction at all.