they are dynamically connected and disconnected
The approach I was thinking, is that each input is a register in a multiplexor. The multiplexor common input is the AVR's RX pin. While in idle mode, each module is scanning his multiplexor inputs, waiting for an incoming message flag.When a module has a message to communicate it's 'child' module, it sets a flag (raises or lowers the voltage) in it's own TX. This child AVR is scanning the multiplexor, and when it finds that in a port, the level is raised, it raises it's TX pin aswell, letting it's parent know that he is listening, and so this parent AVR can start sending his serial.