You could. I reported it as a bug, and apparently it was fixed, based on some posts in MT5.
However, I just tested it in JA, and successfully created a DC8 route via BHK, which is a size 2 airport, limited to large aircraft, but with a 3000m runway.
I still think if you can't fly a regular DC-8 route to it, you shouldn't be able to fly a tech-stop DC8 route through it. But that'd mean needing to remove the apparently blanket restriction on size 2 airports to never allow very large aircraft, even if they've got 6000m runways.
So yes, you can tech stop at any airport with a runway long enough.