Wow - thanks for that! It seems to be working well for me in Ubuntu 14.04; I installed G’MIC from the Ubuntu repository and then followed your instructions. Here’s my test result:
After a bit of experimenting, I decided to go with 7 intra-frames into 60fps video for my eight-second exposures. It’s not perfectly smooth of course (I suppose I should try to reduce the exposure length in future), but I’m pretty pleased overall. I’ll probably tweak the tones and darken the night sky slightly before adding some Sigur Rós music. 
What is the frame step?
The commands I used:
$ gmic -w -fade_files source-images-1080/*.jpg,7,0,-1,1,gmic-7/nl1.jpg
$ ffmpeg -r 60 -pattern_type glob -i 'gmic-7/*.jpg' -qscale 1 nl1-gmic-7-60fps.mp4
Where can I contribute to the hot chocolate fund?