Jon,
     You can write a .mat file where you define specific parameters with variable names that you read into the workspace before you run the simulink model and have those variable names as the values for the blocks.

you could also have a model where you dump the output values form some system to a structure in the workspace and apply that structure value to the input at every time step.

Though i am not positive the latter would work in the desired fashion.

Post more information ifyou need more help.