I must apologise to my maligned steed; she does indeed wake the auto-throttle up if it gets too slow in VNAV PTH. Depending on whether the first speed constraint has been passed it will occur at either -15kts or -10kts but it does not remain indefinitely in THR HOLD and will engage SPD or tell me that thrust is required if the A/T is not engaged.
I've never actually seen this, only "drag required" but that was no excuse for me being lazy and not looking in the book before posting.
I will take myself out behind the bike sheds and give myself a thorough talking to.