Over rugging can have negative effects and I'm not a fan of rugging just for appearance. But I imagine a light, breathable fly sheet would have little effect, especially as it will be removed at night.
Flax, or cold pressed flax oil is a good supp. Biotin is a necessary nutrient which can be lacking in a horse's diet, so may need to be supplemented, but if the horse is getting adequate green forage, it's probably getting enough biotin without. But there are many other nutrients that are most likely lacking or imbalanced. Copper & zinc are 2 that are generally deficient & particularly relate to coat colour & quality.
Coat and hooves are extremities which reflect the health of the whole body. So *generally* I'm not into specific coat or hoof supps and favour overall balanced nutrition. This means it's a good idea to do at least a basic diet analysis, to work out what the horse is already getting & what's needed. I subscribe to an independent diet analysis program/service called FeedXL.com which is great value and help. I'm sure there are many other similar services if that one doesn't suit.
Ensuring the diet is healthy & nutrition well balanced will prevent sun bleaching to a large degree. There is a discussion on FeedXL at the mo regarding supps to darken coats tho.... starting with a well balanced diet. Apparently molasses in large doses(2 cups per day) is effective. **But this is not a healthy supp to give, as horses aren't designed to cope with a lot of sugar.... of which also effects coat & hooves badly. Sunflower seeds(at least 2 cups) are another thing that is suggested. They have a lot of fat in them & if you have an 'easy keeper', it was suggested that gamma oryzanol (an extract from rice bran oil) may be a good alternative.