Just got the latest revision from the fab. Using a low side MOSFET switch does indeed turn the power usage down to unmeasurable levels. There are a few considerations.
You must disconnect the trigger pin by setting it to an input (and setting it back to an output when turning the sensor on) or else it will find GND through that pin and consume power.
Also, the sensor doesn't like being connected to VCC first without getting GND. Several of the available datasheets warn against doing this, with unspecified consequences. Well, one consequence is that the echo pin stays high through the first ping after wakeup, resulting in a short read. I've found a sequence of wakeup, short delay (20 ms), and a sacrificial ping resets the sensor to a working state.
The board has been tightened up some for v1.3, but the enclosure remains the same external size as the length is dictated more by the battery dimensions.
There is also now a trace from VIN to an analog pin, to track battery voltage. It will report out the current battery state when in the parked state every minute by flashing red for the number of integer volts, and green for the first decimal place. When voltage drops to less than 2, it will stop working and flash red three times every few seconds, reminding you to change the battery until it dies.