Comment 16 for bug 974765

For reference, I will post my solution as well.

First, I undid the divert performed by ubiquity via:
$ sudo rm /usr/sbin/update-initramfs
$ sudo dpkg-divert --rename --remove /usr/sbin/update-initramfs

Then I was able to generate initrd images for all kernels via:
$ sudo update-initramfs -c -t -k all

And then updated grub as usual:
$ sudo update-grub

I have since pulled down a kernel via "Update Manager" and update-initramfs is now being triggered as intended by /etc/kernel/postinst.d/initramfs-tools, which now generates working boot configurations.