I don't think it really matters which one you use in this case. I'd probably go with a trigger, because I dislike complex constraints, but that's just a personal preference, not something that's got solid theory behind it.

The better solution would be rebuild the database and normalize it standardly. You already mentioned you won't have the opportunity to do that, so it won't matter. You're stuck with duct-taping it together, and which color of duct-tape (constraint vs trigger) is really unimportant.