Running a liveCD image is not exactly trying out Gentoo ...
Why don't you just install and use video vesa which should be supported by everything? Mount of local hard drive partitions can be done at boot ... if the partition does not exist you just get an error but the boot process should continue.

Ed: just read the link you posted ... Gentoo is different - you set it up. You install the software you want ... you decide if X runs or not ... don't install anything hardware specific like nvidia drivers and things will work on any machine. I suggest you manually start X after login (if you decide you are using X) ... also setup grub and fstab using uuid references.
Whats more likely to happen even if you install hardware specific stuff is an error is thrown when you try to boot on another system and it does not work. Shutdown and plug the USB drive back in the original system and chances are everything will work again
Also probably worth using genkernel rather than manual kernel config ... when you get to kernel setup steps in the handbook.