What I have done in this situation is create a mix project that contains the migrations and schemas (and usually some of the common interaction functions, like ones to generate useful changesets) and then use that as a dependency in all the individual projects.

During development, I replace the usual git path to that library in the mix dependency with a local path, e.g. this:

 {:my_database, git: "git@git.somewhere.com:path/to/my_database"}

becomes

{:my_database, path: "../my_database"}

Then changes during devel to the schemas gets picked up in further calls to mix compile without having to commit, push, mix deps.update etc. Very handy! Then before committing into the feature branch of the application repo(s), I revert the mix.exs change (git stash is pretty handy here), so that the path version does not polute the app repo.