Preventing under-voltage from ESD strike to ground in floating circuit?
What happens to the voltage seen by the MCU between VCC and GND
  when this circuit takes a positive ESD hit to the bottom terminal of
  SW1?


Not a really good way to tell without knowing more of the physical construction of the circuit, but it looks something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Air is in the giga-ohms+ range depending on the humidity and has a few pF's of capacitance to fF's depending on how far the metal conductors are apart. Because the capacitance matters a lot the air and the physical construction will determine the return current path.

My intuition says: odds are having the diode there will sink the current to the ground plane which will be more inclined to take the current in most cases. But if not, you just burned out your MCU port. 

A better way would be to have some kind of chassis ground and a shielded enclosure.