Yep, even though hook wire diameters can be close, so they'll fit in a mold, the hook eye diameter can be a lot different. I've modified a lot of my molds, and some only needed the area for the hook eye to be enlarged. As 21xdc said, high temp silicone is your friend. I buy some that is made by Permatex at an auto parts store, that's good up to around 700 degree's of intermittent temps, and that works very well.
Not all high temp silicone is the same, so read the labels to find those with ratings for temps that work well with molten lead. 600 to 700 degrees.