No, a custom success message is not possible. An error message you can raise using the throw statement (.e.g. throw 'My error message';). This sets also the error HTTP status code.
The question is why your OData service itself should send a custom success message. If the call was successful is indicated by the also by the HTTP status code (depending on the action you have done). The consumer layer can itself decide what success message fits best.
Regards,
Florian