Ive been thinking about this. I have used atmel avr micros before, and recon it wouldnt take too much to hack one up to do this. They set the baud rate with the equasion:
UBRR=((f_clk/(BaudRate*16))-1)
f_clk is crystal speed in hz, and answer is rounded to a whole number. So One of them could do 8192 baud, and be interfaced direct without the max232 to the main cpu.
They can also access sdcards, example code here:
http://www.avrrepository.com/samplecode ... /main.html
Code could be compiled to support the fat filesystem on the sdcard and the device could be installed in the ecu and keep sending log requests and save the response to the sdcard.
if an sdcard sensor with a card present switch was used that could be wired to a digital i/o line so that when no card is present the logger is not active. That way you could just plug in the sdcard and drive, or unplug it and use a pc.