Since i guess there is still no progress on this issue i started to implement my own solution.
At the moment it's an absolutley quick-and-dirty proof of concept, however it shows that this can be done rather easy. Since all of SPPB Code is obfuscated it took me a few tries to get it going, but since Joomshaper has access to the full source code, this can really be done in a few hours.
Process is simple:
-> Replace the two Buttons for Frontend/Backend with an iFrame of the Backend PageBuilder
-> Hide the Toolbar with CSS, so that the Save/Copy Functions of SPPB cannot be triggered
-> Write a function, which triggers SPPB Save as soon as the "Save Article" Button is triggered, wait until the Save is Done and continue with Article Save.
Works great and saves so much time when dealing with Article Integration:
https://screenpal.com/watch/cZV3bJVJw8S
Code of my Concept here - Please don't judge the Spagetti-Javascript. It's only a Proof of Concept.
https://file.io/dm3p7sFGNpEe
(Note: Can be installed as a standard Plugin in Joomla 4, However it has to be loaded LAST to work properly)
Please forward to your developers - I hope, this makes matters quicker and easier? :-)