Appending files to initramfs image - reliable?

I'm modifying a bunch of initramfs archives from different Linux distros in which normally only one file is being changed. I would like to automate the process without switching to root user to extract all files inside the initramfs image and packing them again. First I've tried to generate a list of files for gen_init_cpio *without* extracting all contents on the initramfs archive, i.e. parsing the output of cpio -tvn initrd.img (like ls -l output) through a script which changes all permissions to octal and arranges the output to the format gen_init_cpio wants, like: dir /dev 755 0 0 nod /dev/console 644 0 0 c 5 1 slink /bin/sh busybox 777 0 0 file /bin/busybox initramfs/busybox 755 0 0 This involves some replacements and the script may be hard to write for me, so I've found a better way and I'm asking about how safe and portable is: In some distros we have an initramfs file with concatenated parts, and apparently the kernel parses the whole file extracting all parts packed in a 1-byte boundary, so there is no need to fill each part to a multiple of 512 bytes. I thought this 'feature' can be useful for me to avoid recreating the archive when modifying files inside it. Indeed it works, at least for Debian and CloneZilla. For example if we have modified the /init file on initrd.gz of Debian 8.2.0, we can append it to initrd.gz image with: $ echo ./init | cpio -H newc -o | gzip >> initrd.gz so initrd.gz has two concatenated archives, the original and its modifications. Let's see the result of binwalk: DECIMAL HEXADECIMAL DESCRIPTION -------------------------------------------------------------------------------- 0 0x0 gzip compressed data, maximum compression, has original file name: "initrd", from Unix, last modified: Tue Sep 1 09:33:08 2015 6299939 0x602123 gzip compressed data, from Unix, last modified: Tue Nov 17 16:06:13 2015 It works perfectly. But it is reliable? what restrictions do we have when appending data to initfamfs files? it is safe to append without padding the original archive to a multiple of 512 bytes? from which kernel version is this feature supported?