How do the various tools to convert a system image to f2fs work? Recently I've been trying to install a f2fs compatible rom to my old and slow nexus7. I got as far as flashing a new recovery image and reformatting the device partitions to the new file system, but after trying to install a couple of supposedly compatible ROMs (cm11 and slimkat) and having the tablet stuck at boot I had to revert to the stock image I was running. After some googling it seems I missed a step: "converting" the ROM image to f2fs. There are tools to do that, so I was wondering how do they work actually?
Looking at the tool described in this howto it seems that it's a simple matter of replacing the boot.img with somethink compatible with f2fs and doing a bit of text search and replace on the unzipped image, replacing the strings

format("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/system", "0", "/system") 
format("ext4", "EMMC", "/dev/block/platform/sdhci-tegra.3/by-name/APP", "0", "/system")
mount("ext4", "EMMC", "/dev/block/platform/msm_sdcc.1/by-name/system", "/system") 
mount("ext4", "EMMC", "/dev/block/platform/sdhci-tegra.3/by-name/APP", "/system")
mount("ext4", "EMMC", "/dev/block/platform/sdhci-tegra.3/by-name/APP", "/system")


with

run_program("/sbin/mkfs.f2fs", "/dev/block/platform/msm_sdcc.1/by-name/system")
run_program("/sbin/mkfs.f2fs", "/dev/block/platform/sdhci-tegra.3/by-name/APP")
run_program("/sbin/busybox", "mount", "/system")
run_program("/sbin/busybox", "mount", "/system")
run_program("/sbin/busybox", "mount", "/system")


respectively. (The strings may vary for different devices).

I've tried unzipping a cm-11 and a Slimkat image and the source strings are actually there so the above should work, but I had no such luck for a lollipop factory image, which after unzipping has only .img binary files.