So this is how far I got: https://community.keyoxide.org/api/users
This API lists all the users on the instance. Could be a good starting point.
Isn't there a user-specific API endpoint? Well yes, but: https://community.keyoxide.org/api/users/yarmo returns
404 not found ☹️
Apparently we need to give the ID: https://community.keyoxide.org/api/users/1
This is going to make it a bit complicated. Either the identity claims need to include the user ID which isn't great because the claim should just be https://community.keyoxide.org/u/yarmo.
A better solution would be to take the "all users" endpoint and then filter the JSON to find the right user. Dynamic JSON traversal is technically possible but isn't supported yet in the DOIP libraries. Could be added with relative ease.