Moonlight is good, it can paint you some foreground. Don't put the moon itself in your shot.
Use your in camera NR, it takes a 2nd exposure with the shutter closed to get heat/light spill from in the camera (and hot pixels) and removes that from the original image.
Otherwise take your own black image and do it manually later in photoshop, if you're taking quite a few at the same time length.
As rjd mentioned there's a program to stack shorter (30s to 5m, etc) into one shot to get a full swirl. I think things get odd with really long exposures - heat issues on the chip.