Actually, you might be able to...but most discount codes can't be applied to any sort of a held itinerary.
Cheers.
I haven't looked at the rules for this one, but generally codes are only good for roundtrips
Edit: Rules
http://www.aa.com/aa/i18nForward.do?...=promotioncode
It actually says one way or round trip, so multi-city doesn't seem to be a qualifier.