It sounds quite feasible. Series-connected relay coils arranged so that there wasn't enough current to operate either until some other condition was met were quite common in old telephone switching equipment.
If you connected a relay coil from each phase to ground, then a ground fault on one phase would result in the two relays for the "good" phases being operated. N/O or N/C contacts could then be wired to provide whatever form of signaling is needed.
