@Slowmo,
Much better explanation, thank you.
I like the original idea of using a port pin to power the transceiver. I would just use a resistor divider off one pin at say 0.8 x Vdd, that should cover the range required. Some micros have a DAC built in that could do just that. Considering the actual on time over a year, the waste of the divider is minimal. Also, the time spent re-initializing and powering up (a millisecond or two?) should be negligible compared to the 1-5 minute wait time between transmissions, so why not shut everything down?