I have a friend who tried to cross the border on the road between Van and Khoyl a few years ago. He was stopped by the police on a section of the dirt road in the mountain and was not allowed to cross the border, as it is not open to non-Turkish or non-Iranian people.
There are only two borders open to foreigners between Iran and Turkey. The first is the Gürbulak–Bazargan border near Dogubayazit (Turkey) and Sahabat (Iran) - it is by far the busiest.
The second one is Esendere–Sero and it is farther south. I know someone who got stuck at this border during the winter and was not able to cross because of the snow, so you better make sure it is open if you're planning to go that way during the winter.
I personally biked only the road going through Dogubayazit. I thought this was a truly beautiful region (stunning views of Mount Ararat), but the road going through Esendere-Sero is supposed to be even more scenic.
Has anyone biked the road between Esendere and Sero? Can you provide us any input, descriptions, or advice?