If I'm understanding correctly, you take all frontends out of HAProxy's config, so that HAProxy basically queues requests in-memory until you put a frontend back into circulation? That seems like a much simpler solution for when you have migrations that do not take very long (and enough memory for HAProxy to spool those requests).

Would you be willing to add a minimal example HAProxy config to your blog post that demonstrates your setup? I've been meaning to try out HAProxy but the documentation is so dense I've had trouble getting started. Your use case sounds like a really good reason to give it another go.