How to make nginx wait while proxied app is loading, like Heroku?

We've moved from Heroku to self-hosting for various reasons.

One aspect of Heroku that I've not yet been able to reproduce though, is when our app is restarted, the site immediately starts giving 502 bad gateway errors until the app was finished loading.

On Heroku however it would hold the connection, so the browser will just spin until the app is finished loading, and only then return the response. Is there any way to configure nginx to behave similarly?

Note the "possible duplicate of..." comment below has no accepted answers, and the one upvoted answer doesn't help for REST calls.