Grub should include a microcode initrd if one is present to facilitate early boot intel microcode updates.
Arch Linux already does this. Here is their patch to grub: https://git.archlinux.org/svntogit/packages.git/tree/grub/trunk/grub-intel-ucode.patch
I'm not patching this into Gentoo.
If you can convince upstream to accept a patch, I'm happy to backport.
*** This bug has been marked as a duplicate of bug 579674 ***