Float switch as emergency cutoff for well pump
My recommendation is to build a "float pan" under the run of Cu pipe in question.  As Ed Beal suggested, don't depend on the sump pump as a diagnostic tool.  The  pan only needs to be an inch or so deep, as you expect it to be bone-dry all the time.  Put a float valve on the pan, and tie into well pump power as planned.

BTW, I didn't come up with this on my own:  central air conditioners' air exchange handlers have this sort of arrangement, to avoid disaster if the drain line clogs.