Hi all,
It seems that the main issue is in the physical layer. that is, data conflict when multiple device try to send data simultaneously.
You can try with an external transistor (at the TXD pin)with open collector/drain configuration, so that you will have the wired AND configuration. As the signal get inverted at the TXD pin one more inversion is required at the receiving end also.
Of course the above mentioned solution will help to prevent the hazard in the physical layer. In the protocal layer addressing scheme must be implemented.

The above mentioned solution will have an impact on the baudrate. For high baud rate application this is not suitable.

Any comment on this approach..

Jawahar.