No, you can't. set_param() is used to change the "Parameters" of the model or block. It can not be used to change the dynamic output of a block.
What you want to do sounds like fault injection for debugging purpose. It's like cut a line on a circuit board and feed a different signal temporarily. I am afraid you can't use set_param() for that.
You might have to put this fault injection capability into your design. In your Stateflow chart, define a condition and a third variable. Use this condition and variable to "cut the line".