Gotcha, but I’d avoid it if I were you (there is a bunch that can go wrong, as you know).
Is it something that could be achieved with the current APIs?
Anyways, for Machine apps, both release_command (code) and rolling strat (there’s no blue-green) are driven client-side by flyctl (code). So, if you have a new strat that you want to impl or customize the existing one, it is pretty straight-forward to do so (don’t quote me on it, I’ve never had to do it ;)).
For regular apps, the deployments are handled server-side by Fly, and so short of them implementing a new strat or modifying an existing one, I don’t see how it would be a worthwhile endeavour…