Okay, I figured it out. See, it's simple. You just
Generate a key in one format
Convert it to another
Create a JSON document with a very specific and kind of undocumented format
Create a new account by making raw API calls, because there is no client to do it for you
Call a barely documented endpoint that returns some almost-but-not-quite correctly formatted JSON to put in that JSON document
Strip some characters from one of the fields in that document
and boom, DID:WEB and true decentralization.
Should be easy for anyone to do, right? Way easier than just picking an instance and clicking sign up.