Any reason why a serial connection might interfere with a setWatch for a contact on other ports?

I think you need a P-channel MOSFET - which would allow you to switch the positive voltage to the SIM800 rather than GND, and then a resistor to turn it off, and a diode to drop the extra voltage. It's a shame it gets so complicated when the two voltage levels are different and the device isn't 5v tolerant...

Something like this:

Battery V+  ---------+--------+---------------------­-+
                     |        |
                    +-+       |
                    |-|       |
                    |-|       |
                    |-|       |
                    +-+       |
                     |    +   |
                     |    +---+
                     +----+
                     |    +---+
                     |    +   |
                     |        |
                     |        +------------------+  SIM800 PWR
                   +-+-+
                    \ /
                   --+--
                     |
                     |
Espruino ------------+




Espruino GND  ----------------------------------------­+  SIM800 GND

It seems some SIM800 modules have a PWR pin that can be used to switch them on and off though, which would be so much easier.