Controlling AC mains using Arduino and removing control easily
I solved it by using a SPDT relay instead of SSR. The N/C contact is attached to the manual switch control. The uC then puts the relay in on state to turn the output off. That way even if uC failed, I could still operate it manually. There are corner cases, of course.

BTW, you might need a DPST or DPDT switch for mains control. One pole directly to relay, the other to uC.

It's kind of relay logic, actually. Think of it as staircase wiring (e.g. http://electricaltechnology.org/2012/11/stair-case-wiring-circuit-diagram-or.html).