Yield sense does just that.  It has a scale in the elevator at the turn.  The grain pushes on the scale as it goes by.  Then they have a system to measure the test weight too.

You can get a agleader to match field averages, but spatially not nearly as accurate.  Also if you have 2 different hybrids, you need to calibrate for each variety.  I have seen test plots done with YM and weigh wagon.  Some variety's were off 40 bu/acre.  Just saying.