First you must get the antiquated ideas out of your system.
The DFCS is always "on". Engaged takes on a new meaning with the 737NG.
While the system may not be actively controlling the flight controls for navigation purposes things like Speed Trim and Track calculations are never disabled. The Flight Director is actually telling you what the Auto Flight system would be doing if the Autopilot were engaged or coupled. It is always doing the calculations unless you pull the breakers. When the Autopilot is coupled in any mode the Flight Director is showing you what the system is doing to the flight controls. They are no longer separate systems.
The Auto Throttle is a separate computer but it is fully integrated into the DFCS in a way that allows it to command Pitch changed (Through the FCC's) as well as throttle position. If you select N1 you are limiting it to commanding throttle position. In speed modes it actually "coordinates" with the FCC's to manage the best Throttle/Elevator/Stab positions to achieve the desired result.
Therefore your question is not relevant to the system as designed and engineered. Thrust hold is more of a misnomer used by many to explain a variety of modes depending on previous experience. Some use the term interchangably with what is called "Clamp" in other aircraft and some use the term to mean EPR management.
Thrust hold simply means that the Autothrottle will maintain the desired N1, moving the throttles as appropriate to do so.
Any time the switch is in the Arm position and any mode selected the computer will do what's needed in coordination with the FCC's to make your wishes come true.
I'm glad you got another reply from another respected pilot's forum. I'm sorry you couldn't get the answer you liked from the manuals.
Now if we could just get Boeing off of the ATA 104 "Simplified English" kick and give us manuals that make sense...
