ESD from plastic enclosure to PCB in floating systems
The best possible condition for an electronic enclosure is a faraday cage (a box made of metal). Electric fields on the inside of the box will be close to zero. The air functions like a capacitor and if the ground of the PCB is tied to the box, both sides are zero. No dangerous electric fields.

Mounting a PCB in a plastic enclosure poses risks. The first one is charge buildup. If a net charge is on the outside of a plastic box, the inside is not zero and electric fields and charges that build up on the outside can transfer to the inside (if the plastic is not ESD safe). There is no where to tie a ground and the air functions like a capacitor, on both sides of the board. 

Either use ESD safe or conductive plastics or make the inside of the box conductive. An ESD safe plastic will not build up charge on it. The biggest danger of a plastic box is one that has friction (triboelectric charging) on surfaces when some one moves it, the plastic could build up large amounts of charge on the inside or outside of the box. Either that or build a shield around the PCB on the inside of the box. Copper tape would also work to some extent.


  Can ESD events occur between the inner surface of the enclosure and
  PCB through air?


Yes, ESD can jump through plastic (I've seen myself with an ESD gun) and plastic can collect charge (unless its ESD safe)


  Would it be dangerous to place SMD components on the bottom layer?
  Their pads will be exposed to air, and the distance to the plastic
  surface will be shorter than 5mm.


It won't matter which side of the PCB in the plastic box the components are on, because the charges could build up anywhere. 


  Is there a general solution to protect PCB pads from ESD? Could one
  use something like Kapton tape?


Kapton makes a nice capacitor when placed between two sheets of metal, and has a breakdown voltage, so use a conductive tape, like copper tape. 







simulate this circuit &ndash; Schematic created using CircuitLab