While a metal plate may be preferred, as most consumer handheld devices have limited internal volume dimensions, the interior of the plastic "shell" could be coated with graphite and bonded to the board main Gnd/plane (remember the dark interior surface of Apple Quadra computer cases?).
One weakness of today's inexpensive consumer handheld devices is their poorly designed AC/DC charger form factor, particularly those with only 2 terminal connector and unshielded wire.
Such "wall warts" don't have an earth ground terminal, which could be tied to the handheld device main ground/plane to give a very low impedance path to earth to discharge ESD events.
In this day and age of "throw away" consumer electronics, the lower the cost of the consumer handheld device, the lower the quality and properties of their plastic case/shell.
Is there an opportunity to develop a standard for portable electronics with requirements for only the use "static dissipative" materials and shielded, static dissipative power cords?
It's doubtful it will happen, particularly now that wireless battery charging technology is starting to enter the market (how about their wall warts and charge surfaces?).