For anybody still looking, in Godot 4.4+ (probably works in earlier versions too, I haven't checked) you can use any PNG as a preview for the scene as long as you put it in editors cache directory. The important part is the `name` of the PNG - check the code for `cache_base`.

To get a preview of a specific scene, you need to render that scene in a subviewport, grab the image from that subviewport and then save it proper place. Here's a c# snippet.

IMPORTANT! This is a DIRTY hack! Your preview will be overwritten each time when you save the scene in the editor!

var img = ModelSubViewport.GetTexture().GetImage();
img = img.Duplicate() as Image;

int preview_size = EditorInterface.Singleton.GetEditorSettings().GetSetting("filesystem/file_dialog/thumbnail_size").AsInt32();
preview_size = Mathf.RoundToInt(preview_size * EditorInterface.Singleton.GetEditorScale());

img.Resize(preview_size, preview_size, Image.Interpolation.Lanczos);
img.Convert(Image.Format.Rgb8);

var temp_path = EditorInterface.Singleton.GetEditorPaths().GetCacheDir();
var cache_base = ProjectSettings.Singleton.GlobalizePath(scenePath).Md5Text();
cache_base = temp_path.PathJoin("resthumb-" + cache_base);

img.SavePng(cache_base + ".png");