I'd think the best way to do that would be something like George's boost gauge project. Something like an Ardueno, that you've programmed to monitor that parameter, sent to a small LCD display.
Are you familiar with programming? While i've only really dealt with developing C++ applications for very specific business needs, I dont know how hard it is to program the Ardueno (I hear its supposed to be very user friendly). The other problem would be deciphering the code coming from the OBDII port. But I have to imagine that some people like TD_d, or the romraider community would be pretty helpful for learning how to read the data. Once you learn what sort of format it's in, you could just code an algorithm to pick out that one parameter, and send the proper output commands to whatever small LCD you decide upon. The final stage would, of course, be making some sort of housing for it all.
As a variation of the above, if you leave your tactrix plugged in, you could use the USB-out to interface with the Ardueno (skipping the need for a sacrificial OBDII wire and deciphering the OBDII code itself), and write the code based on the tactrix output. I'd definitely get in touch with
#td_d though...
