I haven't been out in a few years now, but I did some data acquisition when I did go, and we explored this issue. The officials described what they were using without hesitation. Unless you transmit, you can't mess their system up.
What works against you is vehicle speed and response time. Assuming that you are able to receive their signal (which at the time was a constant frequency IR source, I think), your receiver will only be exposed to that signale for a very short time. Since it is an on/off pulse, you have to see that pulse long enough for your detector to receive it AND filter it AND respond to it (send a trigger into your data acq). At the time, our vehicle speed was 150-175 MPH. I believe the conclusion was that it probably wouldn't work, so it wasn't worth the time or effort.
You see, they have the advantage of using your car to break their beam. The longer your car or the slower your car, the longer the beam is broken.
As for GPS, I've done some playing around reading the serial output from a Lowrance receiver. The standard output can only be accurate to 2m because of the output number of digits of precision and the update rate on "retail" systems always seems to be 1 Hz (1/second). Velocity seems to be super accurate, velocity calculated using some earth diameter approximations was not, so I used both the position and velocity signals when I was playing. Also, very few have a "true" compass, and the units that have them are sometimes not accurate. If you programmed waypoints when the course was driven, the heading would be accurate without a compass. Since I didn't have the ability to do GPS when I was assisting, I can't say that this isn't good enough. My gut feel is that a 1 Hz update combined with wheel speed and accelerometer corrections would work real well at Bonneville. If not, I found an evaluation board on the internet somewhere (don't know the link right now) for about $800 that had 5 Hz output. This would provide a nice crossover between car based sensor outputs and GPS sampling frequency as a confirmation of data quality.
Just a few thoughts.