This is great. I've been wondering how to do this, and this is exactly what I had come up with. I'm glad to have confirmation that it's the normal way people do zero-downtime migrations.

I might add one change. You say add a column migration, ship it, then add the feature that uses the column and ship again. I was planning on shipping the code at the same time (or even before) the migration, but having the feature disabled with a feature-flipper until the migration is done.