I'd consider sticking a fast 50mm on a full frame and going with that. http://www.extremeinstability.com/2012-12-13.htm
That is what I got last year with a Sigma 50 on a 5D II. 20 seconds 1600 ISO apparently on this one then stacked the frames.
Too bad coma so bad on that lens and well all the fast canons. This year going to try the Samyang 24 F1.4 on the 6D. Just because it at least doesn't coma. 14 too wide unless something crazy happens. If I had a 50mm that didn't coma I'd probably chose that as the go to lens for this. 35mm probably best on full frame for it. Hard to say but fast lens helps a lot.
Everyone that goes out to do that should at least have a cable release they can lock in and let the camera go in consecutive shooting mode.