@lolo, thanks for your reply.
Unfortunately using the pipelines-json extension doesn't seem to help.
I get the same error:
warning build: argument unused during compilation: '-N descriptors.pipelines-json' [-Wunused-command-line-argument]
I'm also stuck on trying to generate a proper mtlp-json file using metal-source. I have exactly the same error as reported by @peterwong7:
metal-source: error: unsupported binary format