The most interesting thing is, which azimuth?
Southern or Eastern would be possible, but Eastern launches would bring it over central Japan after 800 km, while southern launches miss the biggest population centers.
It is, IMHO, one of the worst locations in the world for a spaceport.
- High latitude
- no really free launch corridors
- No allied country along the launch corridor for gathering telemetry.
North Korea would have to rely on telemetry ships right around the borders of Japan for tracking a rocket in a useful way. Otherwise, they can just fire it up and if it answers one day later (or if china helps them), it is a success.
It is as insane as the whole state North Korea. I can understand if countries would like to shoot down any missile, peaceful or not, which comes from North Korea.