I think these are both good suggestions and valid incentives for players to defend a keep, instead of letting the enemy capture it on purpose.
I would also like to add a suggestion:
Lower the XP/Karma rewards for tower/camp/keep capturing, and increase it for defending.
Maybe the amount of supplies present in a tower/keep can also be a factor in this. If there are fewer or no supplies: a smaller reward for capturing. And the more supplies present, the more reward for the conquerers. The defending team should get a reward not based on the supplies, since supplies themselves already give an advantage for the defenders.