Nigh impossible tbh. But I can see a way.
- Release the movie at a time where the Rocks flashback has already ended in the manga.
- Make it as such that the anime onlies can also watch it (otherwise they won't even make it in the first place)
- Re-animate it or format it again for the anime when that time comes because it's essential to be there just like what they're doing with Mugen train.
Too complicated to match the timing with the manga and the movie also making sure that there's a big gap between when it'll be animated for the anime and the release of the movie so that they can make it as profitable as possible. Very difficult to do with a weekly anime.