That would likely fail due to the two-step OpenWrt sysupgrade, like I said in my original message, linked in the first message of this thread.
Writing rootfs based on the currently running firmware's DTS partition table would likely overwrite the last part of kernel. (OpenWrt first writes kernel, and separately rootfs to the location specified by the currently running firmware)
I propose going via Linksys OEM firmware, or any other way to write a unified "factory" image instead of the two-part sysupgrade image.
(That's why ipq806x required flashing via OEM GUI or TFTP recovery flash)
Sysupgrade force-flashing the OpenWrt factory image might also work, as you can return to OEM firmware via force, so the OpenWrt factory image should be similar as OEM images, and might work.
E.g. for my WRT3200ACM the factory image is 4 MB larger than the sysupgrade image, as the factory image contains lots of 00 padding upto 0x600000 (from the end of the actual kernel to end of the kernel area = the start of rootfs)
just to highlight the possible issue with "normal" sysupgrade, this is the structure of the wrt3200acm sysupgrade image: a TAR archive with separate kernel and rootfs files, which will be flashed separately according to the places specified by the currently running partition structure: