After some significant research, I have figured out how to do this. You make a linear measurement array using precision 1.0000" balls in right angle v-groove with a base plate, and then a matching block at the top. If this is done carefully, you can even reach 1/1000th precision. It looks like this:
Everything has to be scraped and square, especially the foot plate. You will need to pin this and set it up very carefully to ensure right angles. The base plate is inclined so the balls rest by gravity. The array is sized out at an inch height.
The block at the top is used to finish the measurement. For example, if you want a standard length of, say, 53.75 inches, then you use 52 balls and you make a 1.750" block. If you want hundreths precision, then the block needs to be to the nearest thou all around. If you want thousandths precision, then the block needs to be to the nearest tenth all around.
To make the standard get the bar and scrape it all around square except one end, which is matched to the block. Doing this match is the hard part. If you do not know how to flush two surfaces to a thou, talk to a toolmaker to learn a procedure for it.
As an alternative (expensive) way to do this, which is superior, is that there are long length gauge block sets. The typical set has blocks in the following lengths:
20 + 16 + 12 + 10 + 8 + 7 + 6 + 5 = 85 inches
So, you can measure out lengths up to 7 feet using this kind of gauge block set (or combine multiple sets to go even higher). The problem is that a long-length gauge block set sells for between \$1,500 and \$3,000.