Unless some engineer tells you it's okay, I'd stay away from any heavy loads on it, such as genoa blocks. But I've seen it done, like on some of the C&C boats with the sturdy (say about 1/4-inch thick) aluminum toerail with the holes every couple of inches. I just don't know it that's advised in 'the manual'.
Many folks use them to hang fenders from, and for lighter-load blocks from a smaller sail, like a staysail.
Just as a rough rule of thumb, maybe keep your lines smaller than around 3/8-inch, and you would probably be okay. But I'd really have to see it first.
Trust me, I was an English major ;-)