They requested an NEF file so I shot another test image (ISO 200, f3.5, 310 seconds). There appears to be less noise because the image is almost completely black, but after playing with levels you can see the noise is still there (not to mention plenty of magenta discoloration) if you click on the image below.
Sorry I should have thought about mentioning high ISO to begin with, but kept getting distracted for various reasons. The question is though, how does shooting
with a lower ISO work in real situations? You'll now increase exposure time to compensate, and the purple blobs will increase due to the sensor over heating. This pretty much rules out using it for astrophotography (or other fairly long exposure night photography), since Nikon states it's normal. It seems Canon sensors might still be slightly less prone to the problem, although far from perfect. Might be time to consider a film body for this type work?