Just wanted to clear up a little snafu in your terminology. If a crewmember is deadheading then they are indeed on duty and working. Deadheading is when the airline schedules the crew to ride as passengers either at the beginning, end or the middle of a trip to get them into position for their next flight.
I think you might be thinking of non-revving, which is the term used when crew members are travelling on their own time and not on duty.
If you ask a crew member if they are deadheading and they answer yes, then they should still be allowed their normal exemptions.