Not hypothetical for me. I've rehabbed many near ferals. Melissa is correct, ignoring is the best policy. What a feral/near feral/fearful dog needs is to develop a language of intereaction with you, and a set of expectations to hang his hat on, before he can be functional in his new environment.
Total isolation is not the best policy. I prefer using a crate because then I won't be entering the dog's space to care for it. The dog will wear a chain tab and a flat collar with a martingale - I used to have several of these with the quickclose part made of chain - these close faster for the dog's safety. I don't even remember where to get these - now I use premier's, whch I can find anywhere. The dog will be in a large wire crate where it can observe everything, but out of the way. I may start with a cover on the crate, depending on how freaked out the dog is.
To begin with, these dogs will typically react to
everything. I call this "seeing ghosts". I set up a Peace and Quiet routine, and although I do give the dog plenty of opportunity to come out of the crate on its own, about half will require dragging out of the crate the first couple times. This is not as aversive as you might think - remember most of these dogs already have experience getting snared, dragged by the collar here and there, held down and muzzled at vet's offices and shelters. Being "encouraged" with the leash is not so bad. Quick band-aid.
Once out, I'll proceed to the door. Usually, the dog will start seeing ghosts - Border Collies tend to turn into mindless whirling dervishes at this point. I freeze until the brain turns back on again. The first time outside, there may be half a dozen ghost sightings. The next time, half that many. The number goes up and down as the dog is able to assimilate what is normal, then spot things that are out of place, and gradually learn that the environment in general isn't going to kill him, and by the end of the Peace and Quiet phase, that I can be trusted to cue him on what is noteworthy.
Peace and Quiet consists of walks interspersed with crate time. It's crate rest for the brain. By the end of it, he'll have expectations to fall back on about what I expect, his environment, and might even start seeking interaction with me. I might gently praise for eye contact but we don't do anything else during this time.
For the rest of the dog's stay, the Peace and Quiet schedule will remain as a framework, but now more training will start. During leash walks, I'll start changing directions, using the dog's name to build a language of interaction between us. I'll use some corrections - a sharp name call or a refusal to go further, a buzzer noise. If the dog is interested in treats, I'll start using the clicker - but it's rare that a truly feral dog is ready for that level of interaction at this point yet. I expect that more in simply undersocialized dogs. What I'm looking for in a dog at this phase is for the dog to choose to be with me, and to recognize and respond to his name.
We'll do a few days of heightened interaction, then progress to increased formal training. I'll build on the dog's desire to be with me, and for Border Collies things move fast at this point. All the obedience basics are put on, the dog will have his first car ride to the pet store, the dog may see stock for the first time - or for the first time here. This is still overlaid on the Peace and Quiet schedule. By the end of this period, I want to be able to turn the dog loose with 100% recall with distractions, and have some kind of training framework, depending on the destiny of the dog (I'll focus more on clicker for pet and sport dogs, and make sure working dogs are familiar with a more pressure-based approach).
I also want to see the dog seeking attention from other people, too. Normally this falls into place pretty quickly, up to a point - truly feral dogs never really learn to trust strangers without a long interactive period - but all dogs must demonstrate a "coping" mechanism that is appropriate - quiet examination from a safe distance is satisfactory at a minimum. I want the dog to get past avoidance to interaction, even if it's just curiosity.
Usually, depending on what kind of environment the dog came from before, by this time the dog will not seem "abnormal" to the casual observer unless attempts are made to interact. I make sure the dog can retreat from rude people, but I deal with interaction with people just like I did earlier ghost sightings. The dog gets no interaction with me as long as he's acting inappropriately, which now will signal he's on the wrong track. When I see signs that the brain has turned on, I'll use whatever signal we've worked out at that point that I'm pleased.
Most dogs that are simply undersocialized only need one or two reps of this and then they cn proceed to the next level, "Strangers have neat stuff!" I will hand out balls or treats to willing victims and turn the dog loose. the stranger has instructions on looking particularly inviting, while the dog, I hope, expands his comfort zone.
Now about three weeks has gone by and I'll usually see some kind of dramatic personality reversal. I deal with that, and then after that the training is both more normal, and also too customized to address in a single post.