Diodes! Why didn't I think of that? Thanks. I'll breadboard that and see how it works. That would require a pull-up on the common junction and would make the common junction essentially an open collector pin (passive hi and active low).
All the sensors are Arduino Nanos and are distributed over a 70' radius. The central Mega transmits on one TX line with a line driver to drive a common bus to all remote units. That side works OK in the prototype.