Ask HN: Interest in a 0-downtime managed Postgres migration tool?

I recently discovered how much of a hassle it is to migrate off of Heroku Postgres. Both to keep the user from screwing themselves and as a vendor lock-in mechanism there's no possibility of running an external replica for your Heroku Postgres database. Unless I'm missing something after a bunch of Googling it seems like there is a market gap for a tool that allows for a seamless migration away from a Postgres DB that won't allow for replicas.

I imagine that, given some constraints on what queries you allow on your database, you could proxy connections to a managed Postgres database while feeding them simultaneously to a new database. Starting from a snapshot of the old database one could catch the new database up to a live state of the old database and then swap over all connections to the new one.

Does such a proxy already exist? I'd love to know as I could use it. If not it might be fun to build and a good side-project.