If you decide to go back to ride hight measurment, here is another option.
Range Rovers have 4 suspension travel transducers. With so many people replacing their automatic suspension with springs, these should be available cheap.

Even cheaper than dilatometers, you could use linear variable restistors. Install them the same way I suggested with the dilatometers but you would probably want to buy 16 so you could rig up a wheatstone bridge to amplify your response. Basically, you'd have 2 active resistors and 2 non active resistors per wheel. Ivf you are interested in this approach, I'll dig out my instrumentation text books. The nice thing with this approach is that it is purely analog and you could measure the data with a multi meter. Thus getting the cost down to the $100 range all in and still end up with acurate data and still get your body roll through corners etc.

Jack...the data recorder works with analog electrical input...so my advice is to stay with electrical components.
there are such things as linear potentiometers (volume control in straight line form) to use as a primary input.
Measuring the travel in the suspention would be easy with these at each wheel.
Placing a big capacitor across the output would dampen the readings to get a straighter line on the chart.
I think you already have a voltage regulated box to get a good source for the pots.
I think what you are looking for is an overall reading of changes made to the wing each time you come up with a new setting.
This would be easiest if there were no extra steps to take (computations or translations from pressure) to arrive at the answer.
Simplest is best in this case...also...you can leave the linears in place during a race....they would be in parallel with the shock absorbers (from bolt to bolt) and would tell you in real time whethter the setup was good in drafting or open air conditions.
So much for my 2 cents.
I hope you're having fun.
Bob

This is a hard one. If your measurement device allows any real movement then you are going to change the wing settings. Anything besides a load cell or a strain gauge is kind of a kludge.

I think a serious search for a useable load cell if the answer. Not sure if these are suitable but the price is right...
EBay item #7617746512

Linear pot = linear variable resistor.