I agree with Paul Bakker on the why, if you use workflow to send the mails, enhancing the BOR object is probably the best way to do it.
But if all you need is to send a mail, why use worklfows to do it at all? link a receiver function module in transaction SWE2 sending a mail and it will work just as well and not use as much DB tables as the workflow.
Also, Idocs, in most systems, have to tendency to go wrong in the thousands (if not more) which will cause some poor users email to be spammed. a report sending a single mail will do a better job. at least, if you do use workflow to send a mail, add a check function module to stop sending the mails after the 10th (or some other reasonable limit) mail has been sent that day.
