This would be like imitating Spectacle but within the boundaries of Okular's Windows instead of System wide.
But why? What would be the benefit of this? What's the use case?
It would be helpful in case the user is in another DE such as Sway.
Isn't this just 'area selection', then 'copy to clipboard' ?
I think so. If you're in Sway, just use whatever screenshot tool you do have installed. If you don't have one installed... install one. :)