OK, thanks for clarifying, now I was able to install OpenWRT on NAND (and I verified that an update from Luci does not alter the emmc SinoVoip installation). Here are the correct writing operations, if someone needs them (after running insmod mtd-rw.ko i_want_a_brick=1):
mtd write /tmp/openwrt-*-bananapi_bpi-r3-mini-snand-preloader.bin /dev/mtd0 ubidetach -m 1 ubiformat /dev/mtd1 ubiattach -m 1 volsize=$(wc -c < /tmp/openwrt-*-bananapi_bpi-r3-mini-snand-bl31-uboot.fip) ubimkvol /dev/ubi0 -N fip -n 0 -s $volsize -t static ubiupdatevol /dev/ubi0_0 /tmp/openwrt-*-bananapi_bpi-r3-mini-snand-bl31-uboot.fip cd /lib/firmware/airoha cat EthMD32.dm.bin EthMD32.DSP.bin > /tmp/en8811h-fw.bin ubimkvol /dev/ubi0 -N en8811h-fw -n 1 -s 147456 -t static ubiupdatevol /dev/ubi0_1 /tmp/en8811h-fw.bin ubimkvol /dev/ubi0 -n 2 -N ubootenv -s 126976 ubimkvol /dev/ubi0 -n 3 -N ubootenv2 -s 126976 volsize=$(wc -c < /tmp/openwrt-*-bananapi_bpi-r3-mini-initramfs-recovery.itb) ubimkvol /dev/ubi0 -n 4 -N recovery -s $volsize ubiupdatevol /dev/ubi0_4 /tmp/openwrt-*-bananapi_bpi-r3-mini-initramfs-recovery.itb volsize=$(wc -c < /tmp/openwrt-*-bananapi_bpi-r3-mini-squashfs-sysupgrade.itb) ubimkvol /dev/ubi0 -n 5 -N fit -s $volsize ubiupdatevol /dev/ubi0_5 /tmp/openwrt-*-bananapi_bpi-r3-mini-squashfs-sysupgrade.itb ubimkvol /dev/ubi0 -n 6 -N rootfs_data -m