Okay, I think I found the solution. I'm posting it to the mailing list so that (hopefully) when someone else has this same problem and they google for it (like I did) they can find a potential solution without having to sink an excess of time into ripping anaconda apart :-)
The problem lies inside of "upd-instroot" inside the anaconda-runtime package. Inside the "prunePackageList()" function is where it is supposed to find all the RPMs needed for the install environment. It does this by iterating through a list of package names and ls'ing for their associated RPM in your asset directory, with this line: for PKG in $PACKAGES; do PATTERN="${PATTERN:+${PATTERN}|}(^$PKG )" PACKAGEFILES="$PACKAGEFILES $(ls $PACKAGEPATH/${PKG}*{${ARCH},noarch}.rpm 2>/dev/null)" done
The problem was, this custom CentOS-based distro was for an i586-based system, but it was just a first *rough* build to test installation, so I had all *i386.rpm files except for glibc and openssl, which were *i586.rpm.
So, long story short, the errors I was getting were because the i586 glibc was not being pulled into the install environment (the above code only pulls in the RPMs if it is i386).
Hope that helps someone else with this problem in the future, considering that anaconda-runtime proceeds to build bootable ISO directories even if glibc wasn't actually available for the $ARCH it's looking for :-)