I'm sorry. I don't use gentoo so I don't even know why I replied...
I'm inherently distrustful of things that are done for you because you normally get problems such as these... If you manually patch a kernel.org kernel with that patch I listed and then recompile your kernel, it will just work. Period... That and obviously the actuall bootsplash package needs installed and configured...
Maybe a gentoo user can jump in here...