Interesting… Looking inside install.ps1, it appears that it would use plain /windows/ as the platform component:
https://api.fly.io/app/flyctl_releases/windows/x86_64/latest
I’ve heard mention of people successfully running flyctl within WSL2, but I don’t know which exact binary that ends up as.
(I don’t have a Windows machine myself, these days.)
That was good thinking, overall, though…