There's always a "weakest link" and when you reinforce the hitch plate, your transaxle casing becomes the weak link. Best case scenario is you would break bolts. Worst case, you could crack or break the bolt holes out of the casing. The leverage that the tongue weight would put on the hitch is multiplied incrementally as you move away from where it bolts on. So, 500 lbs on the ball would be the equivalent of 1,000 lbs on the original hitch hole. That is if the geometry is right, but you can see how it multiplies. Would definitely recommend looking at it in depth. Also, a sleeve hitch with a ball on it would lessen the stress on your transaxle case.
Good Luck!
Nate