I have to go with Rickair on this one. The 24/7 rule only applies to 121 flying time and not duty time (see part 121.471). You can be assigned to any other duty such as training, deadhead etc. as long as you can find 24 consecutive hours free of duty in 7 calender days before resuming 121 flying.

In terms of duty day, if you were to engage in part 91 flying prior to 121 flying, that flight time must be counted towards the 16 hour max duty day (projected prior to wheels up). However, if you are assigned part 91 flying after 121 flying, you are permitted to exceed the 16 hour duty day as long as any duty past 16 hours is not 121 and that your combined part 121 and 91 duty time is not projected to be over 18 hours. That part 91 flying is still "other commercial flying" and has to be accounted for in rest between duty periods and monthly/yearly flight time calculations

Just my interpretation...