Can you turn the magic wormhole into an API for receiving a JSON payload directly into your magic wormhole ontop of whatever youre running in a fastAPI to route that incoming wormhole listener?
You can also import the wormhole library directly and use its API to run whatever protocol you want. That mode uses the same kinds of codes as the file-sending tool, but with a different "application ID" so they aren't competing for the same short code numbers. https://github.com/magic-wormhole/magic-wormhole/blob/master... has details.
A technique like this is used to do "invites" in Magic Folder, and also in Tahoe-LAFS. That is, they speak a custom protocol over just the Mailbox server in order to do some secrets-exchanging. They never set up a "bulk transport" link.
There is also a Haskell implementation, if that's of interest.
I love to learn about "non-file-transfer" use-cases for Magic Wormhole, so please connect via GitHub (or https://meejah.ca/contact)