Laser on the flat parts of a hex?

A rotary seems obvious, but the software assumes a cylindrical object and will rotate to put successive scan lines at the top of the cylinder, which is not what you want.

So:
Make a fixture to hold the bar with one flat side up
Engrave that side
Manually rotate the bar to the next side
Repeat with the next image …
If you’re doing several bars with the same patterns, make a fixture to hold six (or more!) of them side-by-side, engrave six (or more!) different patterns at once, rotate them while moving each bar to the next slot, and repeat with the same patterns. Once you get half a dozen through the line, you get a finished bar with every pass.

It’s manual, but unless you’re in real mass production, that ought to get the job done.
If you are in mass production, that’s a whole different solution.