Unfilled epoxy provides crush protection but not a lot of tensile strength - you could use glass or carbon chopped fibre filling. A longer steel tube (or aluminium if you prefer) could reinforce the entire stem clamp and headset area and into the steerer below those danger zones. Even if the carbon breaks in those areas the stem is still anchored to the steerer. No I've not done it, I do not own any carbon forks, but if I did then I might - my bad back encourages a higher handlebar position.