How realistic is it to get a walk-up backcountry permit for the Teton Crest Trail in late summer?

Walk-up permits are a real and substantial option on the Teton Crest Trail, but late summer requires effort and flexibility rather than a guarantee of your exact plan.

Grand Teton National Park reserves about one third of all backcountry camping permits for advance online booking on Recreation.gov, while the remaining roughly two thirds are held back as walk-up permits issued in person only, starting one day before a trip begins. Source: https://www.nps.gov/grte/planyourvisit/bcres.htm

Walk-up permits are first-come, first-served at the Craig Thomas Discovery and Visitor Center, Colter Bay Visitor Center, or Jenny Lake Ranger Station, and cannot be reserved by phone or online. Source: https://www.nps.gov/grte/planyourvisit/bcres.htm

The park's own FAQ is direct about late-summer demand: in July and August demand for backcountry permits is high, and the average time prospective campers start lining up is 5:00 a.m. Despite that, the park states that odds of getting a walk-up permit are high if a hiker is flexible, and that a visitor open to seeing what is available and working with a ranger to build an itinerary has a very high chance of leaving with "a great itinerary." Source: https://www.nps.gov/grte/planyourvisit/backcountry-faqs.htm

Flexibility matters because camping zones along the Teton Crest Trail are full almost every night from late July through early September. Rangers recommend being open to a different trailhead, reversing direction, adjusting which nights are spent where, and even starting the same day or the next day rather than insisting on one specific plan. Source: https://www.nps.gov/grte/planyourvisit/backcountry-faqs.htm

For those who strike out entirely at the park desks, the trail's passage through Alaska Basin and Fox Creek Pass crosses into Caribou-Targhee National Forest, where no NPS nightly fee applies and different Forest Service rules govern camping, giving another fallback for assembling a workable route. Source: https://www.nps.gov/grte/planyourvisit/backcountry-faqs.htm

A walk-up permit costs the standard $20 flat permit fee plus $7 per person per night, payable by debit or credit card at pickup. Source: https://www.nps.gov/grte/planyourvisit/bcres.htm

In short, a specific pre-planned Teton Crest itinerary in peak late summer is hard to walk up for, but an adaptable hiker willing to let a ranger build the trip from available sites has a strong chance of getting onto the trail.