The eMMC is just a flash storage device like any block device. You can manipulate it like any device with parted, fdisk, or dd commands. If you boot from TF then ssh into the OpenWRT instance you will see a /dev/mmcblk0. You can manipulate it as you wish without fear of bricking the device. Imagine it as another TF card or hard drive. I’ve repartitioned and dd different images to the eMMC and was able to recover just fine. From my tests and observations the device is just a PC or raspberryPi. From their documentation the boot order is TF then eMMC; so that means if you screw up the eMMC you can still boot from the TF card.
I’ve documented the way to flash OpenWRT to eMMC without Windows and Rockchip dev tools.