As I have mentioned before, and I have been discussing extensively with Tyler (who started this thread) there are two main issues here: cost and standardization. The fact is that in order for any useful information to come out of this type of endeavor, each and every vehicle must be instrumented exactly the same. This ensures that a measurement from one car can be compared to a measurement on the other. This requires that each vehicle have the same instruments, the same shields, the same rack design, and even mounted at the same heights (eg. 11 ft off ground for wind measurement). The problem here is that each car is different and you run into height from car roof issues with some vehicles (like a hummer for example).
The other issue is cost, something that is probably of most concern for most chasers. Most people aren't going to spend $1000 on a set of instruments and pipe to build a rack on their car. $500, though still a bit high for some, is more reasonable, but you run into issues trying to measure all the necessary variables. Temperature and pressure area relatively easy to do, if certain considerations are made and precautions taken. Wind speed and wind direction, and then Relative humidity are both very difficult to do correctly and cheaply.
So while this would be good if it can be worked out, there are some outstanding issues that need to be discussed and decided before someone would decide to go ahead with this.
And it might seem like there would be a good spatial resolution to this data set, keep in mind that there will be a vast majority of observations made in two locations. One location, nowhere near a storm, being made by those that chose their targets wrong, car broke down, or are otherwise having a bad chase day. The other location will be approximately 1000+ measurements within 2-5mi of the meso center/tornado, where almost all of the chasers congregate.