Looks the same way I would go!

Any direction on how to do that with Steem API, without piston?

You need to use the account_update operation and replace the whole posting attribute of the account. With replacing, I mean REPLACE the whole thing. So you take the current posting structure, add the streemian account name to the account_auths, use the threshold as weight and put it into the account_update operation.

It would look like this:

 'operations': [['account_update',
                 {'account': 'fabian',
                  'json_metadata': '{"profile": {"name": "Fabian"}}',
                  'memo_key': 'STM6Gkj27XMkoGsr4zwEvkjNhh4dykbXmPFzHhT8g86jWsqu3U38X',
                  'posting': {'account_auths': [['streemian', '1']],
                              'key_auths': [['STM8EhGWcEuQ2pqCKkGHnbmcTNpWYZDjGTT7ketVBp4gUStDr2brz',
                                             '1']],
                              'weight_threshold': 1}}]],