USART, single master (8051), multiple slave (PIC16F876) - suggestions

We built a machine with a CAN bus connecting modules to a CAN board in a PC.

We had a useful little Laser Beam Power Meter [LBPM] board based on a 
PIC16F876 attached to CAN.  It had a debug serial port.  S/w was developed 
on MPLAB/CCSC C.

The next machine used a Cypress FX2LP (based on an 8051).  It has a debug 
serial port.  S/w was developed on Keil C.

Whaddya know, we need to connect the old LBPM board to the new Cypress 
board.

As I fear this may happen again, I am looking for software to use the USART 
to connect the two.

As I suspect there may be other useful PIC boards from our past that may be 
dragged in, I'd like to use the 8051 as a single master talking to one 
(initially) or more PIC boards.

It's low volume and we do not want to lay out any new boards.

It's quite low throughput.

I accept I am very likely to lose my debug facilty, but that's life.

Both ends seem to support addressing over the USART.

Any simple suggestions?  Protocol?  C source code appreciated :-)

Things we considered but rejected:
a) Put USB target on the PIC board
b) Put CAN on the Cypress board
c) Introduce another interface (SPI, IIC, etc) - they've been used/adapted 
already.
d) Use a USB to CAN adapter box (or make one)
e) Wireless, optical fibres, or any other sort of magic.
f) Etc. 

There's some interesting stuff here:
<http://www.htservices.com/Tools/VBandC/SerialCommunications.htm>

On that page there's a reference to "DF1 - Allen-Bradley (Rockwell)";
if you search around the AB/Rockwell site, you will find descriptions
of their serial protocol.

I would hesitate to write a new protocol; odds are theirs can do what
you need.

GH