You may be able to get dbms output by wrapping your stored procedure call in an anonymous plsql block, where you use dbms_output.get_lines to get the lines, concatenate all lines and return these as the output.
Sample plsql code is in the oracle connector: https://github.com/ako/OracleConnector/blob/efdf613874e0a25db07d656a6073deacf65f530e/javasource/oracleconnector/impl/JdbcConnector.java#L110
You may also want to propose this as an idea for default functionality in the idea forum. We’ll consider it for the roadmap.