I have a number of situations where I want a property to be a custom data type. This might for example be a SocialSecutiryNumber which is constructed with a string but is not a String. I might also use a custom enumerated type where the instance is returned by a factory method.
In the absence of simply adding getter and setter methods with known such as setSocialSecutiryNumberString(String s) and
String getSocialSecutiryNumberString()
is there any way to plug in a custom handler for special types

Read the documentation, custom types.