It’s a bit hard (if not impossible) to provision in the way you described with our current infrastructure.
However, it should be possible with our new “machines” platform. It should give you precise control of where you’re deploying each of your instances.
I don’t think the docs are up-to-date for them, but you can find a lot of info with fly machines --help.
It doesn’t currently work with fly deploy, but we’re working on that. This means you’ll have to update all your machines one-by-one to make any kind of change.
There are also rough edges with it, but it’s improving every day.