There is a fundamental difference between the standard numeric types and fixed-point types.
The storage for standard numeric types (double,single,uint8,...,uint64,int8,...,int64) is a simple block of memory containing the values. The mxCreateNumericArray( ) API function is designed to create only these standard numeric types. (As a side note, it also works for char and logical types as well but this isn't documented).
The fixed-point types are not stored the same as the standard numeric types. Fixed-point types are actually a type of OOP object. You cannot use API functions like mxCreateNumericArray( ) to create OOP objects. Even if you fed it the custom classid for your OOP object, mxCreateNumericArray( ) would not be able to use it to create the fixed-point object.
The only way I know of to create OOP objects from within a mex routine is to use mexCallMATLAB( ) to call a constructor function for the OOP class.
Side Note: In C/C++ there is typically no need for a mxClassIDFromClassName( ) function since you can use the enumerated values directly. E.g., instead of using mxClassIDFromClassName("double") you can simply use mxDOUBLE_CLASS directly in your code.
