Does anyone know whether there's any sort of standard rule for defining
whether the wind is veering or backing? Definitions for pressure trends seem
quite well established, steady, rising, rising rapidly etc). But are there
any equivalent rules for direction?

Presumably one needs to compare the current eg 2-min or 10-min mean
direction with the equivalent value eg 30 or 60 minutes ago. But are there
rules for the time period to use, and when steady becomes backing/veering
and then the threshold for 'rapidly'?