Smart Object PSD Files - Cannot Re-Edit in ACR

You need to keep the concepts straight here. You're confusing raw data with RGB data. They are different animals.

First of all, opening as smart object from Lightroom only makes sense for raw files from a camera. This embeds a copy of the raw file so that you can reprocess that in ACR later.

Opening RGB files the same way makes no sense. Yes, you may want to keep the original as a smart object, but then you can just as well do it in Photoshop. There's no advantage in doing it from Lightroom. It will still be an RGB file inside that smart object. Whether you make that smart object in Lightroom or Photoshop, the contents are identical.

Lightroom and ACR, although primarily raw processors, will process RGB files (PSD/TIFF/jpeg) using a simplified processing pipeline. In Photoshop this was made a lot more convenient with the introduction of the ACR filter, which is not the same as the full ACR raw processor. It's just a way to use the ACR controls on RGB data without having to open ACR itself.

I do not understand why your are trying to do something that you know will not work.  Lightroom and ACR do not support layered files.  Lightroom and ACR do  support RAW, Tiff and Jpeg image files.  ACR can be use as a filter for a layer or a smart filter for a smart object layer.  However, processing processing is different between RAW Image Files and converted Tiff and Jpeg  Image files for they do not have a Camera mosaic image to work from.  There are different types of Smart objects. RAW object have a RAW file in the object and Photoshop will create a temp work RAW file which will open on ACR if you open the smart object.  If the Object is a vector object Photoshop will create a temp Vector file like an AI or SVG file if you open the smart object it will open in Illustrator or whatever application the file extension is associated with.  ACR Preferences can be set to Process Jpeg and Tiff Files.  I don't do that by default for some of my script edit Smart Object and I would need to bypass editing smart object that contain Jpeg and Tiff object file like the scripts bypass RAW Object.   I think your best chance for success scan your images to Tiff or  Jpeb Files and then Place them into you layer documents as smart with your ACR preferences set to Process Tiff and Jpeg files.  I would think ACR would open if the temp work file photoshop created for the  object is a Tiff of Jpejg file.  I do not know for sure I do not process Jpeg and Tiff files via ACR.

This makes sense as the data is not from a camera and PSD is not a raw camera file format or a supported rendered file format. I can't test, however, what happens if you save the scans as DNG and then edit in Lr and open as a SO? Are the camera raw settings maintained? ACR can also work with JPEG or TIFF files if correctly configured.

P.S. I know that saving the rendered files as DNG will not magically turn them into a raw camera file.