Usually the ratings refer to Max lifting capacity at some point in the travel. You may be able to go a little over the rating but not put the implement all the way up. I wouldn't though. 750 vs a 500 rating and you're exceeding the max by 50%.
I've got an old ford rotary cutter that's built like a tank, it has to weigh at least 600#. Plus it's longer than a snowthrower so the weight is suspended out farther and creates quite a large moment that the 3pt has to partially counteract. And I have no problem lifting it. If I don't have the FEL mounted it will lift the front end off the ground when I hit a bump. I'm going to call the dealer and ask them. They're good old farm boys and have probably run these things to the max.