How do I use ODE45 in a Matlab System block with code generation?

I'm trying to write a matlab.System that integrates an ODE using ODE45 in its discrete step function. This system should then be used in a Simulink Matlab System block with code generation enabled (no way to get around that since it needs to run on a real time system). All of this is happening using Matlab R2015b, also no way to get around that.

Here is a simple system that demonstrates my problem:

classdef ODESystem < matlab.System & matlab.system.mixin.Propagates
    properties (DiscreteState)
        x
    end
    
    methods (Access = protected)
        function y = stepImpl(obj, u)
            h = 0.1; % step width
            [~, xs] = ode45(@(t_, x_) ODESystem.sys_f(x_, u), [0, h/2, h], obj.x);
            obj.x = xs(end); % new state at t+h
            y = ODESystem.sys_h(obj.x); % output function
        end
        
        function resetImpl(obj)
            obj.x = 0;
        end
        
        % also a bunch of signal property definition functions, can be seen in the attachment
    end
    
    methods (Static)
        function dx = sys_f(x, u)
            dx = u - x;
        end
        
        function y = sys_h(x)
            y = x^2;
        end
    end
end

And what it looks like in Simulink:

All of this works when running in interpreted mode. Code generation fails however, because anonymous functions are not supported yet in R2015b. So my question is:

How do I parameterize the ODE function with the current input u without using anonymous functions?

Workarounds that I tried and failed: nested functions, storing the current value u in a global variable, a persistent variable, a class property (can't pass a handle to an object method). All of these are not supported for Matlab System objects in Simulink Matlab System blocks.

[Robert Heedt]

I found a workaround: Don't use the "MATLAB System" block in Simulink. Instead use a "MATLAB Function" block. In the functions code I manually instantiate the matlab.System object, store it in a persistent variable and call the setup, step, ... methods. Code generation works this way for some reason.