I can simply recommend to put the code into it's own function which would create a layer around it and then use coder.extrinsic to declare this particular function as extrinsic and then call this function in the MATLAB Function block.
The function itself should only get inputs and outputs with valid Simulink types. Such errors occur since Simulink cannot handle this special type since the object you get is not handled as extrinsic.
So the additional function should solve this actually.
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