They should install the modules to the kernel that /usr/src/linux symbolic link points to like nvidia-kernel and alsa-driver.
The ebuild was designed in a way that the running kernel is used by the I2C Makefiles rather than have some logic to specify the kernel. This is fixed in my I2C / lm-sensors 2.8.0 ebuilds.
I2C 2.8.1 and lm-sensors 2.8.1 are both stable, so I guess I can close this.