I'm looking for a way to serially communicate with a multi mesh Arduino slave network
It's not obvious what the exact query is but answering the subject line question:

You can easily multidrop connect multiple serial / RS232 COM ports by having all slave RX inputs and the master TX output on one line, and all slave TX outputs connected by an OR network to a single line to the master's RX input.   The master 'talks' to all slaves at once. Each slave talks to only the master. Only one slave can usefully talk at once. Message destination assignment and slave-talking arbitration is carried out in software. 

As shown below, the OR network at each slave can be as simple as a diode with cathode (bar side) to TX line and anode to common line to master RX.  A single pullup resistor (or an internal pullup in the master on the RX line) pulls the line high when all slave TX lines are high.

This diagram shows how simple multidrop communications can be.
NB - As shown the diodes polarity is for true RS232 with logical 1 = low = negative.
When connecting directly between UART TTL lvel (5V/0V) outputs reverse the diodes.  

Also, add a pullup from +5V to the Blue RX line to the master - 10k to 5V is probably OK.


From here - Multidrop network for RS232.

When a slave TX pin is high its diode is "blocked" and it does not affect the common slave to master level. When any slave TX line goes low the common line is pulled low via its diode and the master sees the low. Normal RS232 COMs can occur between slave and master.

When the master sends all slaves see the transmission at once. A basic protocol must be established to allow messages to be handled by the intended recipient. This could consist of, e.g. sending an address word + command word or words. You can have a single slave act on a message or all slaves or a sunset, based on your addressing system.

The slave to master direction allows only one slave to send at one and you must ensure either that only one slave ever sends at a time and/or that collisions are able to be resolved.

Despite the obvious limitations of such a system it can work very well and is cheap and simple to implement. If there are many slaves the loadings should be checked so that e.g. the master TX is able to drive all slave RXs at once. As the circuit is unidirectional it is easy to add a basic buffer.