btw, @robimarko, how do you think, maybe it is worth to rework uboot config a bit and merge both "rootfs" partitions together before merging IPQ...-backports back in OpenWRT repo?
Anyway I didn't find any way to make it to boot from another partition without access to the booted system or manual intervention through UART.
So this "duplcation" seems pretty useless, if swithing boot partition is only possible through UART and booted system, but it "eats" 35M, which can be used to increase the space available for flashing custom rootfs, and leave remaining for overlay.
// btw, as far as I calculated, for now overlay partition have size of ~30M (0x01ec0000 == 32243712, dividing it to 1024^2 gives 30.75), but df -h on image from /releases says it is only 15.1M O_o
UPD: Btw, I've also summed all the sizes from /proc/mtd and result is 112.75M, while spec AX3600 page on Wiki says there should be 256M.
So, it seems too big space is disappeared to blame it on inaccuracies of all the kind 
(and even if there is a typo and real flash size is 128M, then 15M (128-113) is still too big amount (~10%) to blame it on inaccuracies