Check if the workflow does some Server.log as well. If so you can get the workflow log events using the logEvents property on the workflow token.
The workflow token is returned by the execute method on a workflow. You will also need to pass parameters. A good example of this is the workflow "Start workflows in parallel".
If the workflow only does System.log then the logs are not persisted outside of the filesystem logs (not in the vCO DB) so you cannot get to them. The only solution is then to duplicate the workflow and to include the server.log.