Great.
If you have not read Separate RSB Configurations, Deploying RTEMS and Deployment in the least number of commands then I suggest you do. These posts show the flow for deployment where the vertical software stack is built to a single tar file. The posts shows some of the waf commands that can be used and what they do.
On a Linux host with rpmbuild the rpmspec build target can be used to build specs files for all found build sets. As I mentioned before you need to run rpmbuild. This separations aids integration to CI runner. The Gemini observatory uses GitLab to build RTEMS, tools and networking to an RPM file they can install in the Docker containers they use.
If this approach is followed you would add platformio.py to pkg and a new target called platformio which could build the files PlatformIO needs.
The config.py code will find the build sets and manage any INI files.
Maybe platform.json could be copied to the build output directory?
Note: default deployment builds to a tar file and rpmspec builds to a single RPM file if that packaging is used. The objective of deployment is to have a repeatable process users can depend on to make a working RTEMS with tools, kernel and any other packages like networking packaged in a way they can install on to a suitable clean machine and have the same known configuration.