I think your premise might be not correct, that Sprites are really 3D objects. I think they just share a lot of similarities.
Perhaps you are referring to the Paste Sprite command drawing to screen fast:
This command is not in practical terms a 3D object. It will paste an image (with sprite scale/color/UV applied) to the screen buffer. With an 3D object, this only happens with a SYNC command, and even then the whole scene is drawn.
Hannibal wrote: "There must be a way to paste an object, say a plane, all over the screen, without the sync or the backdrop offf"
I can offer a solution to this problem. You can convert the object into an image/sprite and then paste it that way.
The year one student way to do this is to setup a separate camera, and use SET CAMERA TO IMAGE, and camera masks, and fast sync to create an image of the 3D object intended. Then you can paste the object as an image/sprite and color key the background (default pure black).
A more advanced approach would be to use this example which preserves object transparency:
https://forum.thegamecreators.com/thread/184514
The example includes a simple command method that will create an image. It also includes an ImageKit V2 plugin version that is fast enough to animate the effect.
This method is based on the idea you can take a picture of a 3D object with a black and a white background, then compare the images to calculate the image with a transparent background.
There may also be an undocumented method of simply screen grabbing and preserving a transparent background. Some of the commands have hidden optional parameters. However it has been about 20 years and I only skimmed the details of that conversation.
