Unlike wild encounters, it seems certain stats for trainer encounters are hard-coded rather than generated each time, so random trainer IV's weren't really a thing in the base ROM.
Looking through a trainer editor, it seemed to vary but most trainers were on the extremely low side. Most regular trainers had all IV's set to 0, and they never seemed to go above ~12 at the most, including gym leaders. Giovanni (as a boss) and the Elite Four had IV's all set to 30, and Green ran the whole scale from 0 to 30 over his encounters.