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?