There is no technical reason that the system needs to be 1 way. There are actual products called RS485 repeaters that are probably superior to making the taps off of an RS485 "main line" and driving out to the sub-nets. R.E. Smith at 485.COM has products like this. You can see one of them at this link:

http://www.rs485.com/pmhubx8.html

Michael Karas