Peter: 1N4148 signal diodes are great. If I need to go to schottky I will. BTW, this is Amerika, the Ludite country that refuses to go metric, so 70' (see the tick?) is 70 feet (22-ish meters, and I can't spit that far). The system is connected full duplex, so I only need to control collisions, not directional issues. I agree that a low value of pull-up is important for wire length, local EMI noise rejection, etc. The TX pins can sink 20 ma easily, so the pull-up could be as low as 250 ohms. I'll start with 2.2K (my favorite value from TTL days).
Crossroads: Yes, all the remote TX lines will be combined at the RX pin(s) at the Mega. Your diagram is correct, except the diodes would be reversed with a pull-up.
Eternity: Thanks for the empirical input. All my units are 5V. I'm using 22/4 cable for my serial loops, which can be used as backup for power distribution. All the units have a 20x4 LCD that consumes about 60 ma, so each remote package uses about 120 ma.