Comment by Aloisius

That would probably require cooperation with Apple.

The contact-only mode is authenticated using an Apple-signed device certificate and a signed record of those contact identifiers (as hashed UUIDs) that have been registered for a particular Apple ID associated with the device.

Someone with a Mac can extract those from the keychain (the people behind OpenDrop have a tool to do this), but otherwise you'd need to register a new apple ID, get Apple to register the contact information, register a device of some sort and then do all the key exchanges.