When installing Gentoo again on one of my machines I noticed that the install/rescue/minimal system loads support for USB keyboards (haven't tested that with PS/2 ones) after the keymap selection which forces the user to wait until the timer is up and get the us layout autoassigned and then use loadkeys to load the keyboaard layout of choice.
Are you sure you didn't "mess" with your BIOS USB keyboard settings?
Pretty sure, yeah. Keyboard works both in GRUB on my HD and in syslinux on the minimal iso. So I would assume that the BIOS keyboard config is correct.
Is this still an issue? I believe we've fixed several issues with USB in the meantime.