We solved this "Simulink has no dynamic relay" problem using an SR flip flop and two relational operators. if the input is greater than threshold one, we set the flip flop If it is less than threshold two, we reset it.
This way you can dynamically set the thresholds and thereby create a "dynamic relay" with explicit defined thresholds. The output will be boolean true and false. If you need other outputs, you can connect to a switch and take alternate values (like 1.0 and 0.0 for example)
The beauty of this solution is that you can also use switches to choose between various thresholds without needing to duplicate the relay logic itself, and the thresholds can be set independently (and continuously if desired)