Merge Field Issue Work Around? Need to Send Unique Promo Codes../

Hi guys, I have a client that just had a huge event and promised promo codes to their attendees. With the merge fields not working, we’d have to send 450 broadcast emails to give the attendees their unique promo codes. Has anyone found a work around for this with any other merge field that does work? The first name and last name work but the client doesn’t want to put the code there obviously? This has been down a week and no solution yet. Any code snippet that might make this work? Thank you for any help!