I got instructions on how to inspect an OpenWrt image without having to flash a router with it, here:
But now that I am actually about to try it, I get suck at step 1. Namely, the instructions assume that I have a *.img file when all my images, whether downloaded or created using make image, are a *.bin.
When I try to go through nevertheless, replacing sudo partx -a openwrt-*.img with sudo partx -a openwrt-*.bin, I get:
partx: /dev/loop20: failed to read partition table
Thanks for that encouragement! Last couple of weeks for this OpenWrt setup was the first time ever I used chmod, scp, tar, etc., each one of them taking hours of reading and experimentation while I tried to figure out just the basics (effect of a chance dot or slash etc.). So an advanced project like binwalk will take me some time.