Why do you guys have such a hard time understanding this concept? The 16 hours pertains to flight duty as a required crewmember. If you are DH in the back you are not a required crewmember, but rather just some poor schmoe with a crappy work contract. The FAA and FARs don't care about you unless you are acting as a required crewmember. If the DH was the first leg of your day it would start your 16 hour clock for flight duties.

Your contract should protect you from having to work 16+ hours, but the FAA/FARs don't in this case.