Arch Linux

That's strange. With boot param modprobe.blacklist=nvidia it shouldn't load the nvidia module on its own. Maybe you have explicit loading of the nvidia module in /etc/mkinitcpio.conf or one of the /etc/modprobe* files?
Also /etc/xorg.conf.d/* could contain some advice for Xorg to force the nvidia driver (whenever possible).

So I've managed to get it partially working. I wasn't clear on the install instructions as I thought the symlinks for the libGL files were provided as part of main packages.

After looking into it, it soon became obvious that mesa-libgl and nvidia-libgl packages ONLY provide symlinks (and nothing else) to the real libGL/GLES/libglx .so files which are contained in the main mesa/nvidia packages (I cannot speak for catalyst as I don't use it).

After realising that, I was able to force pacman to remove the nvidia-libgl and lib32-nvidia-libgl packages and then install the multigl-libgl script with:
pacman -Rdd nvidia-libgl lib32-nvidia-libgl
yaourt -Sy multigl-libgl
sudo switchgl NVIDIA/MESA #choose one only

I then followed the instructions on your site to setup GRUB menu configs for MESA/NVIDIA.
However, while this worked for the NVIDIA driver, I had problems with my kernel not loading the 'nouveau' driver when choosing MESA. Regardless of choosing MESA, Xorg loaded the 'nvidia' driver (I guess through autoconfig?), which also seemed to trigger loading the nvidia kernel module to be at that point as well (ignoring my blacklist!)

I could get around this manually by adding systemd.unit=multi-user.target to kernel boot params then:
modprobe nouveau
systemctl start graphical.target

Thank you for this package - I am about to try it for a USB flash drive with a full Arch install.

A quick question - do you recommend doing a:
pacman -Syu --force
when installing 'lib32-mesa-libgl' and 'mesa-libgl' alongside their nvidia counterparts, and then installing your package to control the main symlinks in /usr/lib{,32}?