We won't be back-porting this and rebuilding+pushing older Ignition versions back up to Docker Hub, no. However, once this gets to release, I'd be happy to put together a derived image that can be built against one of those older upstream images to shim in this functionality. It will look very similar to what is mentioned here.
With this new configuration, the entrypoint script still runs as root and will automatically chown -R (for the most part, there is some advanced handling built into there for symlinks, too) files under IGNITION_INSTALL_LOCATION (including under any bind-mounts or volumes in the tree) to the target UID:GID specified. It will not modify permissions, just ownership.