When I have dealt with heater controllers that use SSRs, I have used a shunt trip breaker upstream of the SSR wired to a fault contact on the controller. If there is a PLC in the mix, I might also wire a PLC output to trip the breaker in case I detect anomalous temp signals.
The shunt trip breaker is more compact than a contactor in many cases and you need circuit protection anyway.
I like this idea, this would save panel space as well. Thank you for the input.