Re: 7300LE + TwinView = no accelerated OpenGL
Problem solved. Thanks.
Running the nvidia install script directly seems to have done something good that Gentoo's ebuild installer did not. (I'm not sure exactly what.)
I ran emerge afterward to fix the package metadata, and this did not break the driver.
So, for posterity, "emerge -C nvidia-drivers; sh ./NVIDIA-Linux-x86-VERSION.run; emerge nvidia-drivers; eselect opengl set nvidia" worked for me.