You can use the clicker theory without using a clicker and without using food. You just need to find a different marker and a different reward. So a vocal marker will work in lieu of a clicker. And for a reward, try excited verbal praise or physical praise. Since it is a Lab, he probably likes toys or balls, you can use that as well.
With guide dogs you run into different situations, though. When I train dogs, either for competition or daily manners, I like to have a happy excited dog. With guide dogs, they are required to be a little more sedate in order to do their job and fill the role of a service dog (essentially, blending into the back ground and just being a tool that won't draw attention to their handler).
With that in mind, what I would try would be to get the dog to walk in heel (the heel that is required of a guide dog, like you described) and reward with calm verbal praise, maybe some light physical touch. Do a couple reps of that, then release the dog and reward with physical play, ball, tug, chase, etc. So they learn that when they are calm and walking appropriately, it will earn them a release to play like a Lab pup wants to play. Then, when back at work (ie. heeling) they go back into the calm mindframe.
The "turn and jerk" method can be adapted to positive training, the difference being that you give the dog a verbal warning before you turn away and you don't jerk the leash as you do. So if the dog forges ahead, you might say something like "hey, this way!" then turn and walk the other direction, praise when the dog catches up to you and resumes heel position. The physical motion of you turning and walking in the other way is the only pressure to the leash that is needed, no jerking of the leash, just your body moving forward that applies the pressure, and always with a verbal cue before the change of direction.
Now, that is what I would do if I got to make the rules. But guide dog schools have their own sets of rules, so I wouldn't want to go changing their teaching strategy without first consulting with them. That said, there are guide dog schools that do not allow the use of training collars (chokes and prongs) that are, in my opinion, much more reasonable. I personally would not train for someone that insisted I use a prong or choke, they are damaging to the dog both physically and mentally and follow a very outdated method of training. In my opinion, service dogs should behave appropriately without the threat of a choke or a pinch. If you train a dog with a training collar (choke or prong) then it is very difficult to wean off of that, often times when you switch to a flat buckle collar afterwards, you have no control over the dog. They have learned that the training collar is what delivers the correction, so if the trainin collar is off then all bets are off and you, as a trainer and handler, have nothing to fall back on. Service dogs should not be working with training collars on, they should be solid, stable and doing their job regardless of the type of collar, not controlled by a choke or prong. But again, that is delving into the realm of personal opinion and you have to follow the rules of the guide dog school you are working with, at the risk of the dog you are raising being a wash out based on their criteria if you don't. It's a touch place to be, one that I wouldn't put myself in.