RS-422 spec includes a receiver common-mode range of +/-7V (spec)
and 10V (survive) which a single-ended diode would bind up.
At the transmitter, such clamping would be OK.
You'd like to have some idea just how transmitter, receiver
and earth grounds are related, before you take any shortcuts.
If (say) your transmitter and receiver are running off power
supplies that are galvanically isolated, there might be no decent
current return from "earth ground" to either of the "protected"
devices. If you don't have a good low impedance current
loop then your clamps may not be as effective as you imagine.
The threat current loop has to be closed tightly, so you need
to know where it comes from in order to return it there as
cleanly as possible, preferably through a route that does not
involve the electronics at either end.