Wow...quite a "windfall" of responses! I thought this topic might generate a lot of thought and discussion. So, as the OP, I'd like to thank everyone for their interest. Here's what I have gleaned from the discussions:
When you are actually being propelled my machinery, you are unquestionably a power driven vessel. That is clearly required by the COLREGs.
Your lights and dayshapes indicate to vessels in sight how they should expect you to behave in head-on, crossing, and overtaking situations.
If you have your engine running, ready to put in gear, and intend to operate your vessel as a power driven vessel, then you should display power driven vessel lights and shape so others will know what to expect.
If you have your engine running to charge batteries, but you intend to remain under sail and act like a sailboat then you are under sail you should display those lights/shapes. Again, other boats within sight then know what to expect from you and can act accordingly.
My logic here is that boats with electrical propulsion available at the flip of a switch would not be considered power driven vessels unless they are actually being propelled by the engine. Yet power is even more readily available that with an engine running ni neutral. So, the intent of the operator is key.
Not under command. The COLREGS don't list what "circumstances" justify use of the NUC lights/shapes. The NUC lights and shapes signal to other boats that you are not going maneuver as required by the rules. Other boats have to take that on face value and act accordingly. If there is an incident and the not under command circumstances are due to operator negligence, carelessness, bad judgment,etc..., that is for the court to decide. In the case of a solo sailor sleeping, the not under command issue is separate from the failure to stand an appropriate watch issue. He could be correct in his use of NUC, but incorrect in his watchkeeping and in taking action outside the rule when in extremis in order to avoid collision.
I saw the reference to BOATUS online course saying that if the engine is running you are powerboat...I have sent them an e:mail asking them to clarify the apparent conflict with the COLREGS. I'll let you know what they say.
The COLREGS establish a framework that sets requirements for lights, shapes, and behaviour that help set expectations so we can operate safely. In all cases the captain has to decide the most prudent course of action, and not set false expectation. And, of course, you can use the radio and talk to the other guy.