Thanks so much for your reply, and the suggested circuit. It seems that I will be best served, for the moment, to try the circuit with the NPN transistor, but substitute the low-threshold voltage p-channel mosfet you suggested. I will order some parts and report on my results.
In researching this subject, I found some good discussion in an Arduino forum post here. That forum thread also points to more discussion of battery level sensing for wireless sensors at:
» Zero-power measurement – part 2 » JeeLabs
Both good resources if somewhat over my head. The JeeLabs post suggests this variation on the circuit:
Both discussions refer to using the internal voltage reference of 1.1V, either calling it Aref or bandgap. Not sure how to access that, so may wait for another day and some more research before I try it.