Hello again cg0,
I must add the following: if you need to add any block to the initialization function (called once at the beginning), you can use the standard Simulink block Initialize Function. You can find it by typing "Initialize" in the model, then selecting it, or from the Simulink library browser (slLibraryBrowser) - under Simulink/User-Defined Functions. You can put inside this special subsystem the GPIO output block, with the constant you need to set in the initialization. You can delete the state writer that comes with the standard block.
View from Top model - Initialize subsystem:

View from inside the Initialize subsystem:

Note: You'll most likely have to set "Unspecified initialization detection" to "Simplified" - for the model to build. You will get an error otherwise.

Hope this helps,
Razvan.