Examples of taking existing initrds/initramfss, unpacking them, modifying a few bits, and putting the whole thing back together:

<h3>initrd</h3>
<ul>
<li><a href="http://www.alexonlinux.com/opening-and-modifying-the-initrd">good writeup</a>
<li><a href="http://wiki.openvz.org/Modifying_initrd_image">nother writeup</a>
<li><a href="http://www.flaterco.com/kb/slackware.html">nother one</a> (search for "mount options")
<li>[2007] <A href="http://www.slax.org/forum.php?action=view&parentID=367">adding httpfs to SLAX</a>
<li>[2009] <a href="http://wiki.tinycorelinux.com/tiki-index.php?page=Remastering">changing TinyCore's initrd to add modules</a>
<li><a href="http://bosslinux.in/demo/remaster-initrd/">TODO: fill in link description</a>
<li><a href="http://www.nongnu.org/lpi-manuals/lpi-201/html/ch02s03.html#id290544">Linux System Administration Study Guide</a>
<li>for ones that are gzipped-cpio-archives, you just have to loop-mount them and edit them in-place!
    <ul>
    <li><a href="http://www.boot-land.net/forums/index.php?s=c6a18200ae56a7580eac7efdf3aaef04&showtopic=5514&pid=42636&mode=threaded&start=">modifying Knoppix's linuxrc</a>
    <li><a href="http://bosslinux.in/demo/remaster-initrd/">modifying BOSSlinux's linuxrc</a>
    </ul>
<li>squashfs
    <ul>
    <li><a href="http://www.symantec.com/connect/forums/running-headless-linux-and-modifying-initrd#comment-2343821">unsquashfs-edit-mksquashfs</a>
    </ul>
<li><a href="http://linux.die.net/man/8/mkinitrd">mkinitrd</a>
    <ul>
    <li><a href="http://www.iospirit.com/blog/article/15/Linux-HowTo-Setting-up-and-booting-Debian-Linux-from-Soft-RAID1/">boot from software RAID</a>
    </ul>
<li><i><a href="http://www.google.com/search?hl=en&client=firefox-a&rls=com.ubuntu%3Aen-US%3Aunofficial&q=mount+loop+mkfs.ext2+OR+mke2fs+initrd&aq=f&aqi=&oq=">more</a></i>
</ul>

<h3>initramfs</h3>