With a properly operating dual master cylinder if one end (front or back) fails the other end will still be operational. If it's the front brakes that fail, just using rear brakes can be exciting, but they will stop the vehicle.
Brake warning lights work fine is you have a slight leak, brake line or fitting, or a wheel cylinder. However, the only time I've had a failure was all at once when a rusty brake line failed, on a '64 F100 and it had only a single master cylinder, and a warning light would not have worked. It didn't take me long to find a suitable dual master cylinder and convert it to a dual system.
That said, I think Tony's idea of a float level switch is a good one. There may be a better way of installing floats in your Wilwood master cylinder, though. You can source a float from a vehicle in a salvage yard, a lot of them use floats, some use sensors that would require additional electronics. I believe that Hondas is one that uses floats.
Or, get something like this, easier to install although not as slick looking, due to the wires coming straight up out of the top:
littelfuse_reed_sensors_59630_datasheet.pdf.pdf
You can get them for less than $7 each.
59630-1T02A - Littelfuse - 59630 1T02A FLOAT SENSOR ROHS COMPLIANT: YES (newark.com)