It's already possible to specify icons in the .gdextension file, see #277.

Since this is somewhat out-of-band and GDExtension meanwhile supports icons to be specified as part of class declaration, we could support this as well:

#[derive(GodotClass)]
#[class(icon = "res://path/to/icon.png")]
struct MyClass {}

Ideally this would also allow any AsArg<GString> expressions -- not just literals, but also constants or global functions.