I tried getting by without a permit but could not talk my way past the polite English speaking guard at the gate.
If you do not have an official permit for Yadong county then you have to go all the way back to Shigatse (VT spelling: Xigaze) to get one. Permits are not issued in Kangma county or in Gyangze.
There is a way around the military checkpoint but you'll need a good off-road vehicle such as a jeep. Go east from Kangma toward Nyeru and take a left turn through the mountain pass that proceeds southwest in the direction of Samada. I am confident this detour would succeed without a permit as there are no further checkpoints on Highway 204 all the way to Yadong county.
In Kangma it is not difficult to hire a vehicle to take you east to Nyeru and beyond. The dirt road to Nyeru passes a small village and some ruins along the way, and most drivers are willing to stop and let you get out to take photos on a whim. This activity should not cost more than 100 RMB, and the reward includes views of snow capped peaks and pastoral Tibetan countryside off the beaten path. It takes about half an hour to reach Nyeru from the town of Kangma.
It's a good idea to purchase a Tibet provincial atlas that shows all of the small roads in detail. The driver you hire will likely not have one and so it just makes it easier to show them where exactly you wish to go. Good map books are available at Xinhua bookstores throughout China. I bought my Tibet provincial atlas in a Beijing bookstore for only 12 RMB.