it seems wasteful to dedicate almost 20% of the SRAM to this array alone.
As long as you don't run out it doesn't matter if it's wasteful, it's not like you can pass the unused RAM on to your kids 
You can't use the internal flash, you can use external flash/EEPROM or the internal EEPROM but all those options are LOT slower than RAM.
The only consideration for transmissions that are lengthy is the impact on the module that is supposed to be doing other things too, such as power readings.
Hardware serial is interrupt driven, if the code it written well there will be almost no impact on the main task.
ping interval between 10-30 seconds.
Then using the scheme I suggested above you would transmit a (for example) 0 after every 1Hz pulse, no slaves will have that address so they will all ignore it. Every 20 or 30 seconds you send a valid address and read data.
Rob