I think the "Photon" is an Arduino-compatible board. Amirite?
My interpretation of the OP is the switch either turns the SSR on or lets the Photon control it. The SSR is switching an AC load.
Why not make the switch an input to the Photon? That would be easy. Then the code knows when it is allowed to switch the SSR.
Look at the standard schematic symbol for a SPDT switch and try to think about how that can meet your original request.