The easiest thing to do when your system is nonlinear (as in your case) is not to use a State-Space block. Instead, if possible, try decompose your system into foundational blocks like Integrator, Add/Subtract, Gain, etc. blocks.
This way, you can easily feed that output signal back into a Product block without having to do any "hacking" with the workspace.
In fact, you could just take those matrices and implement the state-space math using matrix operations and one Integrator block. That is, xdot = A*x + B*u and y = C*x + D*u. Then, you could use an Assignment block to keep tuning a particular element of A using that one output element.
- Sebastian
