Nah, that can't be. You are always fine without a seperate /usr. I have MBR and disk has doslabel, no GPT.
Ideally no programs in /usr should be necessary to boot the system. My best guess now is that the problem is /usr/lib. It may be that / can be mounted and that than the booting process at that point becomes more complicated and that /usr/lib is needed. That would explain it, but as I said,at the moment it is just a guess.
If it were up to me essential system libs would be in lib instead of /usr/lib but possibly udev needs libs in /usr/lib/ Or udev (and maybe eudev). Udev originally handled removable media, and because it did this well it made sense to have udev also handle the harddisks. When udev originally was designed to do its job when the system was already running, and only later got the task of mounting the harddisk it seems less weird that /usr/lib is needed.
These are my thoughts at the moment, but I may be completely and utterly wrong.
My first priority is get my system up and running and I will experiment after that when I know my system is sane.
I now have a working system on my external harddisk including xserver, but there is still a lot to do. I have to configure openbox and get myself a panel (probably lxde-panel). After that I have to explore portage a little more, and when everything is fine I'll put funtoo on my hard disk. Only then is my external drive free to experiment with disklayout filesystems an yes or no initram.
When I find out the gory details I will post them here, or, possibly if it is interesting and usefull write a wiki page.