domain alias customehostname.co.za on the workaddress.co.z mail server... piss easy
However I assume they (or you) would like to actually trace what mail has been forwarded etc... tough one. Depends on the mail server you're using. SmarterMail I know can forward mail (and either keep or delete the incoming mail) to a different address, not entirely sure it has the ability to alter the subject though.
But, again, it depends on the mail server software you're running, if both domains are on the same mail server or not, if not, are they running the same type of mail server or what... etc...
Else you could write your own add on (AGAIN DEPENDING ON THE MAIL SERVER) which would then intercept the message, change the subject, and let the mail server handle the forwarding....
Did I mention this depends on the mail server?