"Is there a list of [graphics] objects in The MathWorks documentation which are not "sealed?" "
No, there is no such list.
"I am trying to create a custom class to overload the MATLAB image object "
Can you say a little more about why you're trying to overload the image object? Perhaps you can already do what you want and you're just not aware of how to do it, or perhaps this could be worthy of an enhancement request?