Is There a Way to Measure Relative Torque on the Car?

That would seem to be the way to go. Wireless is the key. I haven't gotten the price on the KA Sensors DSTS, but their stuff isn't inexpensive.

It looks like Caleb at Izzy Racing also has something in that vein:

On-Vehicle Dynamometer, Wireless Strain Gauge / Torque Transducer - Izze... https://youtu.be/jPsKwfQU4KQ

Why isn't this more widely used?

I'd say its down to cost as you've mentioned, plus using it for tuning will still be a little problematic as there are many other environmental factors out on the road which will require many, many runs to average out.

This is why dyno tuning is great for the bulk of the job, it eliminates many of these factors and you can focus in on how your changes are effecting engine torque. The caveat it that a dyno tune should always be checked on the road, as road manners and drive-ability are more important than peak numbers I reckon.