Here is an example of using the 8-15mm Fisheye I took. Although I did use a long exposure to get star trails. Hopefully it is still informative for you.
I also have used the TS-E 24mm II for astrophotography and have quite enjoyed it, however I can definitely understand wanting the EF 24mm 1.4 opposed to it for that specific use. Here is an example although I do not have any Milky Way shots with the 24mm.
How did you manage to fit a filter in front of the fisheye? Or you just used bulb mode in near darkness?
No filter used, just a ~15 minute exposure. It was extremely dark though and was difficult to compose easily. I end up illuminating the center focus point and looking through the viewfinder with both eyes open and focusing to infinity (with my eyes, not the lens) to superimpose the focus point on what I am directly looking at straight ahead with my other eye, if that makes any sense. It is usually surprisingly accurate and definitely eliminates having to waste several long exposure shots just getting things composed well.