Hello Lorenz,
As you have already figured out , the Action API provides you with only the messages and failed keys if any.
Post action execution , you can always execute a retrieve or retrive by association , to get the latest buffer snapshot , which of course would include the changes that you have made in your action.
If you want to ensure that users have explict control on execution of your "fucntion", then of course , you should model it as an action on the BO.
The parameter is_parameters is an IMPORTING parameter. You CANNOT use it to export anything back from the action. For importing , you can of course have any structure to use as the is_paramaters , which you model as the action parameter structure which modelling your BO action.
Form an external entity the only way to interact with a BO is by consuming the BO services and you are bound by the BOBF standard interfaces. Any and all data you require needs to be modelled as node attributes ( persistent or transient ) and fetched using the RETRIEVE, RETRIEVE_BY_ASSOCIATION or QUERY services.
Regards,
Indranil.