I wasn't able to find much about narrowband UVB treatments on this forum so I thought I'd post our experience for the record, in case anyone else is looking at it. I think there is some older technology called PUVA that some have used, but I believe this is notably different.

When my son reached about 90% hairloss his dermatologist suggested we try narrow band UVB. The reason it wasn't suggested before is because there wasn't enough exposed scalp.

The treatment is in an upright booth, I think very similar to a stand-up tanning setup. It is exclusively UVB, as opposed to UVA, so the risk of skin cancer is not significant compared to actual sun exposure. He would go into the booth and cover everything except for his scalp. I'd dress him in pants, socks, long sleeve shirt, gloves and we'd position a towel over his face. He also needed to wear special goggles. The treatments started at 10s every 2 days and increased in duration and intensity up to 2.5 minutes every 2 days. We went for approximately 4 months, maybe 5. There were a couple of instances where we were away and he went as long as 6 days without treatment. Visits to the treatment center were always very quick in and out.

The premise of UVB is that it is a local immunosupressant. I know it is used in psoriasis and other skin issue.

I believe we went approximately 1.5 months before we started seeing vellus hairs over the entire scalp. After 2 months we had colored terminal hairs mostly covering scalp with a few bare patches.
After 3 months we had a full scalp of terminal hairs.
after 4 months we had a lot of hairs longer than 1" with shorter hairs everywhere else
it wasn't long after that that he had his first haircut in nearly 2 years!

we stopped treatments at the end of July, and went until mid december without finding any new outbreaks. Since then it has been a lot slower progression than his initial outbreak, with the system responding quite well to spot treatments with clobetasol.

I think the UVB treatment made a big difference in response. I definitely noticed that the areas that were slower to come back were the spots where he was holding the towel on, and weren't getting as much if any exposure during treatment.
One other nice side effect is that his scalp got a nice tan, which helps in overall appearance while waiting for growth to happen.

The treatment wasn't overly cumbersome for us, as we found a dermatology clinic near home that was able to offer the treatment and had operating hours we could work with. For a person who lived outside of the city, or did not have a conveniently located treatment center it would be difficult due to the frequency of treatments.

Hopefully this helps someone!