My newly installed system hangs right after these lines:

INIT: Entering runlevel: 3
* Starting metalog... [ ok ]

Is there a way to initiate interactive boot of some sort in order to troubleshoot the issue?

I had the same problem and i resolved it recompiling and reinstalling kernel and modules

Run gentoo with the option "single" in your boot line. You'll get a command line you can solve this from. Perhaps your metalog configuration is incorrect? Also, if you can locate the problem, report it to the gentoo bugzilla system

if you have modified any module in the default state and something is wrong linux stop loading in the service which module or configuration is wrong (maybe alsa).
try this:
use a linux livecd.
create a directory to mount your gentoo.
#mkdir /mnt/gentoo
mount the root partition in that directory.
#mount /dev/hdX /mnt/gentoo
where hdX is the root partition
chroot your gentoo
#cd /mnt/gentoo
#chroot /mnt/gentoo /bin/bash
Enter in the directory where the kernel source code is
#cd /usr/src/linux
and make the modules install again
#make modules_install
and reboot the system
this should be enought
I hope it to be usefull.