A barking mad policy..... Trainers can be heads down for hours filling out the complicated forms while looking after a new cadet but line pilots can't use a PED. Talented people these trainers to have such superior skills.
I don't think you'll find any trainer worth his (or her) salt that uses a PED to fill in an OGS form whilst conducting training, especially with a Cadet. Sitting in the jump-seat conducting a Line Check, though, well thats a different matter.