... my gut feeling is a lot of damage was being done ...
I very much doubt that any damage was being done. If damage was possible, then P&WC would have advised against the practice.
... used quite regularly in the arctic to avoid blowing the prop seals, by getting low pressure warm oil to the gearbox/prophub.
Yes, holding the prop until the oil has warmed (and become sufficiently viscous) means that the gearbox won't be damaged by what would otherwise be inadequate lubrication.