Our rule of thumb for migration paths is to set them up on an as-needed basis. Even before formal migration paths exist, moderators may migrate to any site, and users may flag questions to request migration. When we see significant numbers of questions moving from one site to another (and not being rejected by the destination), we'll think about setting up a path. Yes, this is pretty subjective. But it works well enough for a system that's already an edge case.
For what it's worth, as I write this, Law doesn't have outbound migrations to any site at the rate of even a single question a week over the past three months.