Most of the time, I find that using a PIC as an intelligent peripheral is easier than using a dedicated chip. Why not use a chip like an 8723 (with two USARTS) as a peripheral? It can then do things exactly as you wish. That device could could easily have a 1K buffer.

More importantly, why do you think you need all those USARTs to network your PICs? I have networked over 40 PICs together using one 18F8723 as a master talking to 18F2321's as slaves - and no extra chips are involved. The master's TXD line is connected to the (hardware) RXD line of all the slaves.
The slaves then send data back using a bit-banged (SEROUT2 port), because the hardware TX port can't drive open-collector. All the output ports of the slaves are tied together, and that goes to the master's hardware serial RX port. The master polls the slaves by sending out an 8 bit address followed by a command and a checksum. The slaves use an interrupt to grab the bytes. If there is an address match, the slaves wait 600uSec and then send the requested data back in inverted (idle high) open-collector mode. Using this mode prevents bus contention. A 22K pull up is used on every slave.


The whole scheme is still working flawlessly, and polls all the slaves in less than half a second. Although I have checksums and retries buit into the system, the only time it ever retries is when I hot-plug a slave. I also reserved address 253 for a "general call" that all slaves receive, but don't respond to. It is used to produce an output on the pins of all devices simultaneously. Address 254 is used to re-address the slaves. The 2321 board is so small there is no room for DIP switches or jumpers for addressing. By hooking up one device (only) to the master, and then sending to address 254, followed by a new address, followed by a checksum, you can change or set the device's address. It is stored in EEPROM and read on every power up.

The only downside to using RS-232 between devices is that their oscillators have to be pretty stable if you want fast communication. Crystals on both sides are a must for speeds greater than 19.2Kbaud.